11Network Architecture: Layers Some slides are from lectures by Nick Mckeown, Ion Stoica, FransKaashoek, Hari Balakrishnan, and Sam Madden Prof. Dina KatabiChapter 7.B and 7.C2Last Lecture We learned how to share the network infrastructure between many connections/flows We also learned about the implications of the sharing scheme (circuit or packet switching) on the service that the traffic receives3This Lecture An Example: HTTP Layering (read 7.B) Link Layer (read 7.C)4Example: HTTP over the InternetUsing TCP/IP and EthernetAppOSR2 R3R4R1R5Ethernet“A” MIT“B”(CNN.Com)EthernetAppOS12346720191817591081213111516145In the sending host1. Application-Programming Interface (API) Application requests TCP connection with “B”2. Transmission Control Protocol (TCP) Creates TCP “Connection setup” packet TCP requests IP packet to be sent to “B”TCPDataTCPHeaderTCP PacketType = Connection SetupEmpty in Connection Setup Packet6In the sending host (2)3. Internet Protocol (IP) Creates IP packet with correct addresses. IP requests packet to be sent to router.IPDataTCP PacketEncapsulationIPHeaderIP PacketDestination Address: IP “B”Source Address: IP “A”Protocol = TCPTCPDataTCPHeader27In the sending host (3)4. Link (“MAC” or Ethernet) Protocol Creates MAC frame. Wait for access to the line. Send each bit of the frame.EthernetDataIP PacketEthernetFCSEthernetHeaderEthernet PacketDestination Address: MAC “R1”Source Address: MAC “A”Protocol = IPIPDataIPHeaderEncapsulation8In Router R15. Link (“MAC” or Ethernet) Protocol Accept MAC frame, check address and Frame Check Sequence (FCS) to ensure no bit errors. Pass data to IP Protocol.EthernetDataIP PacketEthernetFCSEthernetHeaderEthernet PacketDestination Address: MAC “R1”Source Address: MAC “A”Protocol = IPIPDataIPHeaderDecapsulation9In Router R16. Internet Protocol (IP) Use IP destination address to decide where to send packet next (“next-hop routing”). Request Link Protocol to transmit packet.IPDataIPHeaderIP PacketDestination Address: IP “B”Source Address: IP “A”Protocol = TCP10In Router R17. Link (“MAC” or Ethernet) Protocol Creates MAC frame. Wait for access to the line. Send each bit of the frame.EthernetDataIP PacketEthernetFCSEthernetHeaderEthernet PacketDestination Address: MAC “R2”Source Address: MAC “R1”Protocol = IPIPDataIPHeaderEncapsulation11In Router R516. Link (“MAC” or Ethernet) Protocol Creates MAC frame. Wait for access to the line. Send each bit of the frame.EthernetDataIP PacketEthernetFCSEthernetHeaderEthernet PacketDestination Address: MAC “B”Source Address: MAC “R5”Protocol = IPIPDataIPHeaderEncapsulationSteps 8-15 are the same as before …12In the receiving host17. Link (“MAC” or Ethernet) Protocol Accept MAC frame, check address and Frame Check Sequence (FCS) for bit errors. Pass data to IP Protocol.EthernetDataIP PacketEthernetFCSEthernetHeaderEthernet PacketDestination Address: MAC “B”Source Address: MAC “R5”Protocol = IPIPDataIPHeaderDecapsulation313In the receiving host (2)18. Internet Protocol (IP) Verify IP address. Extract/decapsulate TCP packet from IP packet. Pass TCP packet to TCP Protocol.IPDataTCP PacketDecapsulationIPHeaderIP PacketDestination Address: IP “B”Source Address: IP “A”Protocol = TCPTCPDataTCPHeader14In the receiving host (3)19. Transmission Control Protocol (TCP) Accepts TCP “Connection setup” packet Establishes connection by sending “Ack”.20. Application-Programming Interface (API) Application receives request for TCP connection with “A”.TCPDataTCPHeaderTCP PacketType = Connection SetupEmpty15This Lecture An Example: HTTP Layering Link Layer16Network ArchitectureProblem Networks are complex (heterogeneity, distributed, delay, losses, reordering, …) How do we organize a network implementation?Solution To deal with complexity Æ use layering17Layering Layering is a particular form of abstraction The system is broken into a vertical hierarchy of logically distinct entities (layers) The service provided by one layer is based solely on the service provided by layer below18Layering: Our HTTP ExampleNetworkLinkTransportThe 4-layer Internet modelApplicationHTTPIPTCPEthernetEnd-to-End Layer419Who Does What? Link Layer: Delivers data from one end of a link to the other Network Layer Routes packets and delivers them to their destination Transport Layer Provides useful abstractions: stream, message Can provide reliability Can provide congestion control Application Layer Your application: HTTP, FTP, etc. 20Where are these layers? Link and network layers are implemented everywhere The end-to-end layer (i.e., transport and application) is implemented only at hostsApplicationTransportNetworkDatalinkApplicationTransportNetworkDatalinkNetworkDatalinkPhysical mediumPeer-layer communicationRouterHostHost21Encapsulation A layer can use only the service provided by the layer immediate below it Each layer may change and add a header to data packetdatadatadatadatadatadatadatadata22Interface Higher layer calls lower layer e.g., Link_Send(this_data, this_link)Lower layer uses an up-call function to inform the higher layer of data arrival e.g., Network_Handle() 23Multiplexing in the Internet Many possible applications, transports, and link layers But they all use IP at the network layer InternetLinkTransportApplicationIPEthernetPacketradioTCP UDPTelnet FTP HTTP24This Lecture An Example: HTTP Layering Link Layer525Link LayerProblem:Deliver data from one end of the link to the otherNeed to address: BitsÆ Analog Æ Bits Framing Errors Medium Access Control (The Ethernet Paper)26Sending bits Bits Æ Analog Æ Bits Receiver needs to detect the value of the bits Manchester Encoding: each bit is a transition Having a transition in each bit allows the receiver to synchronize to the sender’s clockTime011 1027Framing Receiver needs to detect the beginning and the end of a frame Use special bit-patterns at the beginning and the end of the frame Bit stuffing is used to ensure that a special pattern does not occur in the data E.g., pattern is 1111111 (7 ones) Bit stuffing: whenever the sender sees a sequence of 6 ones in the data, it inserts a zero (reverse this operation at
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