BU CS 101 - Computer Networking (26 pages)

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Computer Networking



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Computer Networking

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Pages:
26
School:
Boston University
Course:
Cs 101 - Comp & Appl
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Computer Networking 20 1 The Internet Hourglass Model The physical layer is how machines are physically connected to each other Applicatio n NV TCP Transport InterNetwor k Physical HTTP FTP TFTP UDP IP Token Ring Ethern et Wi fi 802 11 20 2 The inter network layer The inter network layer is what allows all machines on the Internet to communicate with each other Notice that many different protocols exist above below IP but they all converge to IP this is how all computers can communicate over the Internet HTTP NV TFTP FTP Application TCP Transport InterNetwor k Physical UDP IP Token Ethern Wi fi 802 11 20 3 Checksum We need a simple way of letting the receiver check if this frame is intact How can a recipient tell if a frame has errors from the frame it has received CheckSum construction algorithm Sender Add all the bits in the frame which will yield another string of bits we can calculate this quickly CheckSum Packet verification algorithm Receiver Add all the bits in the packet and compare to the checksum in the packet Match Packet is good No Throw the packet away Can we fix flipped bits using a checksum 20 4 Motivating the Transport Layer The Physical layer allows us to move bits between two computers The Internet Protocol IP builds on top of this and enables data to travel across networks on the Internet However there is a lot of things that IP doesn t do What order are the packets coming in As a sender are there other features we might like Knowledge of whether a packet actually made it to the target Other communication about the target s ability to receive more data Concerns about not flooding over utilizing the network Yet would every network application need all of the above Does it make sense to force this functionality on all network communications What would be the apparent cost of adding such features More bits more data to send time 20 5 Motivating the Transport Layer IP Packe t IP Packe t IP Packe t IP allows us move packets from one computer on the Internet to any other computer on the Internet What if multiple programs want to use the network Which packets go to which application Packe t Web Browser File Transfer Instant Messenger 20 6 Transport The transport layer provides end to end connectivity allows transfer of packets from one specific application on one computer another specific application on another computer provides each application on the connected computer the shared use of the network through a unique interface ports In this context ports are conceptual All network traffic is coming to the machine in the same way but the separation into ports allows different applications to share the network The purpose of ports and the transport layer Each application only cares about its own network traffic and should only receive that traffic Additionally some transport layer protocols provide features to Ensure messages are arriving in order Make sure that all messages arrive at the receiver and other legitimately useful features 20 7 Transport Layer How bytes move from one application on a computer another application on another computer Applicatio n NV TCP Transport TFTP UDP IP InterNetwork Physical HTTP FTP Token Ring Ethern et wi fi 802 11 20 8 Transport Layer Pack et for port 6734 Pack et for port 6734 Pack et for port 6736 Web Browser Instant Messenger File Transfer Through the use of ports packets can be addressed to an individual port on a given machine That port will be associated with a specific program So you can send a packet not only to a specific computer but to a specific program pack et buffe r at port 6734 Transport Layer pack pack et et buffe buffe r r at at port port 6735 6736 20 9 Transport The two most common transport layer protocols are UDP User Datagram Protocol Delivers packets to a particular port on the computer Best effort packet delivery packets get there in order out of order or not at all How do packets arrive out of order TCP Transmission Control Protocol Reliable byte stream delivers packets reliably in order to a particular port on the receiving machine Uses retransmission if data is lost midstream How do you know if a packet has arrived and wasn t lost 20 10 Transport UDP vs TCP Both include sequence numbers to identify the intended order of the packets Both indicate a port number which help direct the packet to the correct application Each protocol has it s own specific header Depending on which you are using that header and your data payload which will then be the payload inside an IP packet which will be placed in a physical layer packet example Why use best effort when you can have reliable transmission What is the cost associated with reliability 20 11 Transport TCP Transmission Control Protocol Includes features for Reliable transportation be sure that the receiver got the data sent Receivers send Acknowledgements ACKs of received packets Flow Control The receiver will tell you how much more data it can receive such that you don t send data faster than the receiver can receive it Congestion Control If packets aren t making it through to your receiver perhaps the network can t keep up slow down don t send data faster than the network can handle it 20 12 Practical Issues applied to the Internet Reliability Will my packets get where they re going How can the communication between physical layer nodes be unreliable How can things get lost if there s a wire there If we have TCP How does TCP make a transmission reliable Is a connection reliable What if the physical layer is error prone say it drops 50 of your packets Does TCP have a breaking point Reliability effects transit in lots of ways Slows us down Increases load on the network 20 13 Application Layer Now that we ve gotten byte to a particular application how do two applications know how to exchange messages Application NV TCP Transport TFTP UDP IP InterNetwork Physical HTTP FTP Token Ring Ethern et wi fi 802 11 20 14 Applications If we can send bits over a network we can transmit anything that we can represent in binary Numbers Characters Sound Video anything If we can connect all computers via the Internet What sort of applications can we enable E mail File Transfer FTP Hyper text Web Newsgroups USENET Chat IRC Instant Messaging w Presence Indication Internet Phone VOIP Internet Video 20 15 Applications Network Applications terminology Server A dedicated computer that is part of a network The hard drive contains files that are served to whatever requests them Could be


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