Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 2620-1Computer Networking20-2The Internet Hourglass Model•The physical layer is how machines are physically connected to each other . . .FTPHTTP NV TFTPTCP UDPIPToken RingWi-fi (802.11)EthernetInterNetworkTransportApplicationPhysical20-3The 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!)FTPHTTP NV TFTPTCP UDPIPToken RingWi-fi (802.11)EthernetInterNetworkTransportApplicationPhysical20-4Checksum–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?20-5Motivating 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 = time20-6Motivating the Transport LayerWeb BrowserFile TransferInstant MessengerIP Packet• 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 ?IP PacketIP PacketWhich packets go to which application !?!Packet20-7Transport•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 features20-8Transport Layer•How bytes move from one application on a computer another application on another computer. FTPHTTP NV TFTPTCP UDPIPToken Ringwi-fi (802.11)EthernetInterNetworkTransportApplicationPhysical20-9Transport LayerWeb BrowserFile TransferInstant MessengerPacket for port 6734• 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!Packet for port 6736Packet for port 6734Transport Layerpacketbuffer at port6734packetbuffer at port6735packetbuffer at port673620-10Transport•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?20-11Transport•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?20-12Transport•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 it20-13Practical 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 network20-14Application Layer•Now that we've gotten byte to a particular application, how do two applications know how to exchange messages?FTPHTTP NV TFTPTCP UDPIPToken Ringwi-fi (802.11)EthernetInterNetworkApplicationTransportPhysical20-15Applications•If we can send bits over a network, we can transmit anything that we can represent in