Introduction to the Internet. Internet Basics. Back in time ARPANET 11/20/10 ENGG st semester, Hayden Kwok-Hay So

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1 11/20/10 Introduction to the Internet ENGG1015 1st Semester, 2010 Hayden Kwok-Hay So 2 Internet Basics The Internet is a that connects millions of of computing devices throughout the world. The Internet is formed by connecting many different s. Inter -Net Distributed management Internet in 2003 What s so special about the Internet? 3 Back in time Before the Internet, each University and military unit has their own Different standards Some of them work only when the computers are ly close to each other Simple point-to-point connections 4 ARPANET Advanced Research Projects Agency Network Project active during early 70s Commonly referred as the predecessor of the Internet A large-scale research effort involved multiple Universities on building s that Src: DEC s PDP-10 Connect computers remotely in long distance Scalable Networking was a niche 5 First operational packet-switch 6 1

2 3 Big Ideas about Internet The end-to-end design principle The core is dumb Layering odular design Division of labor Packet switching Data are transmitted as packets Compared to circuit switching s 7 8 History of Packet Switching Though it sounds trivial now, the concept of packet switching was first introduced back in the time when ARPANET was still at its infant stage. To provide reliable connection in case a is down as long as smart protocol (TCP) is used. To improve the system capacity To make a scalable system 9 Internet Packet Routing Surprisingly, the Internet routing protocol is relatively simple. Each router only knows the general direction on where to route a packet A hierarchal routing idea Example: Send a package from the USA to the address Rm 516, Dept of EEE, HKU, HK 1. Send to HK 2. Postman (router) at HK sends to HKU 3. Router at HKU sends to EEE 4. Router at EEE sends to Rm 516 The simple design of IP routing makes the Internet extremely scalable! 10 Internet structure: of s a packet passes through many s! Tier 3 Tier-2 Tier 1 Tier-2 NAP Visualizing Internet Routes Traceroute A standard tool to look for all the intermediate routers to a remote site Interesting website that provides visualization of routes Tier 1 Tier-2 Tier 1 Tier-2 Tier

3 Layered Architecture (1) Divides the task of communication into layers of smaller modules Each layer has a well defined sub-task Each module has restricted interaction with each other Each layer provides services to the layer(s) above utilizes services from the layer(s) below Uses services of Uses services of Layer 2 Layer 1 Layer 0 Provides services to Provides services to Layered Architecture (2) The module in a particular layer only communicates with its counterpart on the other side of the at the same layer. Any module can be replaced with another module at the same layer as long as it provides the exact same interface. This is the reason, e.g., your web browser works identically regardless of whether you are using WiFi or a wired, or 3G mobile 15 Network Protocol A common language for different s to communicate Protocol defines the format and order of messages being sent and received among entities. It also specifies actions that should be taken by the entity upon message transmission and receipt Human analogies: say hello when you pick up a phone When you are asked what is the time?, you would answer The time is instead of y shirt is white 16 Layers, protocols, and interfaces. Protocol Hierarchies The philosopher-translator-secretary architecture

4 The OSI Network odel Open Systems Interconnection Basic Reference odel OSI 7 layer model A standard layering model Abstract the a into 7 layers Unfortunately (fortunately) the Internet has its own notion of layering Internet layer slightly different from the OSI model But may be mapped back to 5 layers in the OSI model Application Presentation Session Transport Network Data Link Physical Internet protocol stack : supporting s FTP, STP, HTTP : host-host transfer TCP, UDP : routing of grams from source to destination IP, routing protocols : transfer between neighboring elements PPP, Ethernet : bits on the wire How Layering Works? Each layer hides the layers above from any detail about lower layers. Each layer focuses on its core functions, assuming other layers will handle the rest. e.g. A logical connection at layer ack 21 Layering: communication 22 Protocol layering and Data Each layer takes from above adds header information to create new unit passes new unit to layer below source destination Ht HnHt Hl HnHt Ht HnHt Hl HnHt message segment gram frame 23 25

5 End-to-End Design Principle In laymen s term: What happens at the end host is what really matters Slightly more technical: Some information are only known to the end hosts, so there should be a distinction in responsibility on what the end hosts should perform and what the should perform. Jump to conclusion: The core should be dumb. Example: Reliable File Transfer While you are uploading your photo to Facebook on your mobile phone, you entered into an elevator and lost your mobile phone connection. Q: which of the following makes more sense? 1. The is smart, so that when the connection comes back (after you exit the elevator), it should retry the upload of the photo. 2. The is dumb, and terminate the connection to Facebook without uploading the picture File Upload (cont d) According to E2E principle, the Internet design picks (2) as the answer. Reason: the mobile phone should not retry the upload because it does not, and should not understand the high-level requirement of the user. e.g. You may decide to redo the upload if it has failed, and if the photo upload was really that important. Or, you may just give up and upload another photo If the (think your mobile phone carrier) retries the upload, AND you perform another upload at the same time, you will end up with two different versions of photo uploaded. Quick Summary Internet is a collection of global s End-to-End architecture allows the core of the be constantly changing without affecting the end nodes Packet-switch architecture allows many more nodes to share the same resource than circuit switch Standardized protocols allow different computers to communicate Protocol stacks provide layer abstractions Administrivia Homework 2 graded Pick up at tutorial next week Pick up at Rm 601 after Wednesday Homework 3 due 24 Nov, 5pm. Final project presentation + competition Dec 1, 10am Get graded and return materials afterwards Final Exam Dec 20, 2:30 4:30pm Open books, open notes Bring your calculator 31 32

6 Introduction The Internet Protocol (IP) layer, as well as TCP, are the two most important protocols of the Internet. Roughly corresponds to the Network layer in the OSI 7-layer model Currently IPv4 deployed on most of Internet IPv6 starting to be deployed in some s Service odel A best effort service model It tries its best to deliver grams, but it makes no guarantees. Connectionless Two parts of service: Addressing Scheme: Allows node in the Internet to be identified Datagram delivery: Allows to be transmitted over wide varieties of underlying s Data Delivery Data are delivered in a best effort, connectionless manner Relax the requirements of the underlying Layering abstracts the difference between underlying s to higher layer -- TCP Data are transmitted using predefined packet. Fragmentation+Reassembly is used to break long packets into smaller pieces if needed. Usually as a result of underlying. IPv4 Packet Header Version: version of protocol. 4 or 6 Length: length including header TTL: Time to live Checksum: checksum of header SourceAddr: Source IP address DestinationAddr: Destination IP Address Addressing Scheme A node on the Internet is identified by its IP address In IPv4 standard 32 bits Usually represented as 4 integers connected with dots E.g A hierarchical addressing scheme Host part + part The higher order bits (bits on the left) are used to identify The lower order bits are used to identify hosts within the Sample Addressing Scheme E.g. 3 IP s x x x The first 24 bits are part Last 8 bits are host part Q: How many hosts can you have within 1? Q: Why 24 bits? LAN

7 Address Classes IP addresses are divided into 3 classes Class A: First bit is 0 7 bits for id 16 hosts Class B: First 2 bits are bits for id ax 64k hosts Class C First 3 bits are bits for id ax 256 hosts Subnetting Problems with the 3 classes classification: any IP address not usable even if the owner of the id does not need all numbers And IPv4 IP numbers are running out! Subnetting allows fine grain division of an IP address in to +subnet+host parts Real identifier = IP & Subnetmask Private vs Public Original IPv4 assumes all hosts on the Internet have their own IP addresses Public IP address Uniquely identify a host on the Internet Three groups of special private addresses for private LAN uses. 10.x.x.x x.x x.x They should not be used publicly. Not unique over the entire Internet One group of auto assigned private IPs x.x Should not be used normally E.g., assigned automatically by OS if no reply from DHCP server IP Routing Each router on the Internet has a limited information about what the rest of the Internet looks like A router understands what is the next hop of a packet based on its routing table and forwarding table Special inter-router protocol that updates routing tables of routers Cons: It is possible to have loop Routes are not always optimal Pros: Scalable` Destination x.x x.x en0 en1 Output Interface en0 en en1 IPv6 The next generation IP standard Designed to address issues in IPv4 IP address running out Security, real-time service support, etc IPv6 Address: 128 bits Even a toaster can have an IP address! x:x:x:x:x:x:x:x Each x is a 16 bits hexadecimal number E.g. Like in IPv4, IPv6 defines many address spaces depending on the prefix of the address Deployment much slower than expected ostly because IPv4 has not run out as fast as expected Network Address Translation (NAT) hides many private IPs from the public internet 47CD:1234:4422:AC02:0022:1234:A456:0124 Physical Network IP protocol runs on top of many different s Common s Ethernet Wireless LAN

8 Ethernet ost common LAN technology Common standards: 10 bps Ethernet, 100 bps Gigabit Ethernet and 10 Gigabit Ethernet Uses a shared medium Originally shared a wire 10Base2, 10Base5 odern Ethernet s use switches/hubs to connect multiple segments AC An excellent example of the general Carrier Sense ultiple Access with Collision Detect (CSA/CD) technology ultiple node on a Ethernet LAN may want to transmit at the same time If more than 1 host transmit at the same time, collision occurs The algorithm to determine when/how to transmit/ retransmit is called is media access control (AC). An Ethernet node do the following thing: It senses the media to see if it is idle or someone else is transmitting (CS) It transmit opportunistically While it transmits, it detects if collisions occur (CD) If collisions have occurred, it wait for a random time and retransmit again. Frame Format and Address Data sent on a Ethernet as frames simple header min 46 bytes Need minimum length for collision detection max 1500 bytes Each Ethernet adaptor has a 48 bits globally unique address. The Ethernet address Sometimes called AC address Denoted as x:x:x:x:x:x Each x is a 8 bit number written as 2 hex numbers Top bits: manufacturer id; lower bits: serial # Need to be unique to avoid name crash within a shared LAN Wireless LAN ost common standard: b bps, a, g bps n -- ~300 bps (not standardized yet) Designed for use in a small geographical area E.g.Signal is not design to travel far Design is similar to Ethernet standard Both are shared medium s Subtle differences between the 2 in terms of the medium Collision Avoidance Although wireless is a shared medium, carrier sense/collision detection is difficult because not all nodes can see all other nodes Hidden node problem Collision happens due to a hidden node that the sender is not aware of E.g. since A,C dont see each other, they may send to B at the same time and collide Exposed node problem A node fail to send thinking the medium is being used when it is not E.g. B is transmitting to A and C wants to transmit to D. C hears B is transmitting so it wait, but in fact, it is ok for C to send to D.