CS155b: E-CommerceAnnouncementSlide 3Slide 4The Physical LayerThe Role of the IP LayerSlide 7Slide 8Slide 9Adding Some FunctionalitySlide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Where to Go NextSlide 20Slide 21Slide 22OSPF Won’t Work Between CompaniesBusiness Relationships Connect the InternetBorder Gateway Protocol (BGP)Border Gateway Protocol (BGP) ConceptsBGP PreferencesReferencesHomework Assignment For January 23CS155b: E-CommerceLecture 3: Jan. 21, 2003How Does the Internet Work? (continued)Acknowledgements: J. Rexford and V. RamachandranAnnouncementProfessor Feigenbaum’s office hours are canceled on Thursday, 1/23.The TA will hold usual office hours on Wednesday, 1/22, from 3-4pm.Layering in theIP ProtocolsInternet ProtocolTransmission ControlProtocolUser Datagram ProtocolTelnetHTTP(Web)SONET ATMEthernetSimple NetworkManagementDomain NameServiceInternet ArchitectureISP 1ISP 2ISP 3NAPcommercialcustomeraccess routergateway routerdial-in accessdestinationdestinationinterdomainprotocolsintradomainprotocolsprivate peeringThe Physical Layer•A network spans different hardware.•Physical components can work however they want, as long as the interface between them is consistent.•Then, different hardware can be connected.dial-in accessEthernet cableserverEthernet switch•Internet Protocol (IP): gives a standard way to “package” messages across different hardware types.The Role of the IP Layermodemaccess pointPPPhubserver10BaseTEthernet……FDDI100BaseTEthernetrouterrouterrouter1. Message is put in IP packet.2. Dial-up hardware gets packet to router (however it wants, but intact).3. Routers look at destination, decide where to send it next.4. Packet gets to destination network.5. Original message extracted from packet.IP Connectionless Paradigm•No error detection or correction forpacket data–Higher-level protocol can provide error checking•Successive packets may not follow the same path–Not a problem as long as packets reach the destination•Packets can be delivered out-of-order–Receiver can put packets back in order (if necessary)•Packets may be lost or arbitrarily delayed–Sender can send the packets again (if desired)•No network congestion control (beyond “drop”)–Send can slow down in response to loss or delayIP Packet Structure4-bitVersion4-bitHeaderLength8-bitType of Service(TOS)16-bitTotal Length (Bytes)16-bit Identification3-bitFlags13-bit Fragment Offset8-bit Time to Live (TTL)8-bitProtocol16-bit Header Checksum32-bit Source IP Address32-bit Destination IP AddressOptions (if any)Payload20-byteHeaderMain IP Header Fields•Version number (e.g., version 4, version 6)•Header length (number of 4-byte words)•Header checksum (error check on header)•Source and destination IP addresses•Upper-level protocol (e.g., TCP, UDP)•Length in bytes (up to 65,535 bytes)•IP options (security, routing, timestamping, etc.)•TTL (prevents messages from looping around forever; packets “die” if they “get lost”)Adding Some Functionality•More guarantees, e. g., that packets go in order, require more work at both ends.•Solution: add another layer (e.g ., TCP)original msgTCPSourceIPoriginal msgTCPDestinationIPhardwareEncapsulationmsgmsgTCPhdrmsgTCPhdrIPhdr(or without TCP)(or without TCP)Transmission Control Protocol (TCP)•Byte-stream socket abstractionfor applications•Retransmission of lost or corrupted packets•Flow-control to respond to network congestion•Simultaneous transmission in both directions•Multiplexing of multiple logical connectionssource network destinationTCP connectionTCP Header16-bit destination port number32-bit sequence number32-bit acknowledgement number16-bit TCP checksumOptions (if any)Payload20-byteHeader16-bit source port number16-bit window size4-bitheaderlengthFINSYNRSTPSHACKURG16-bit urgent pointerEstablishing a TCP Connection•Three-way handshake to establish connection–Host A sends a SYN (open) to the host B–Host B returns a SYN acknowledgement (ACK)–Host A sends an ACK to acknowledge the SYN ACK•Closing the connection–Finish (FIN) to close and receive remaining bytes(and other host sends a FIN ACK to acknowledge)–Reset (RST) to close and not receive remaining bytesSYNSYN ACKACKDataFINFIN ACKACKtimeABLost and Corrupted Packets•Detecting corrupted and lost packets–Error detection via checksum on header and data–Sender sends packet, sets timeout, and waits for ACK–Receiver sends ACKs for received packets•Retransmission from sender–Sender retransmits lost/corrupted packets–Receiver reassembles and reorders packets–Receiver discards corrupted and duplicated packetsPacket loss rates are high (e.g., 10%),causing significant delay (especially forshort Web transfers)!TCP Flow Control•Packet loss used to indicate network congestion–Router drops packets when buffers are (nearly) full–Affected TCP connection reacts by backing off•Window-based flow control–Sender limits number of outstanding bytes–Sender reduces window size when packets are lost–Initial slow-start phase to learn a good window size•TCP flow-control header fields–Window size (maximum # of outstanding bytes)–Sequence number (byte offset from starting #)–Acknowledgement number (cumulative bytes)User Datagram Protocol (UDP)•Some applications do not want or need TCP–Don’t need recovery from lost or corrupted packets–Don’t want flow control to respond to loss/congestion•Fraction of UDP packets is rapidly increasing–Commonly used for multimedia applications–UDP traffic interferes with TCP performance–But, many firewalls do not accept UDP packets•Dealing with the growth in UDP traffic–Pressure for applications to apply flow control–Future routers may enforce “TCP-like” behavior–Need better mathematical models of TCP behaviorGetting from A to B: Summary•Need IP addresses for:•Self (to use as source address)•DNS Server (to map names to addresses)•Default router to reach other hosts(e.g., gateway)•Use DNS to get destination address•Pass message through TCP/IP handler•Send it off! Routers will do the work:•Physically connecting different networks•Deciding where to next send packets (HOW??)Connecting NetworksAOLAutonomousSystem (AS)EarthLinkAutonomousSystem (AS)WorldNetAutonomous System:A collection of IP subnets and routersunder the same administrative authority.Interior Routing Protocol (e.g., Open Shortest Path
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