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 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34– 1 –15-441Internetworking - ForwardingFeb. 13, 2006TopicsTopics•What's an Internet?•IP forwarding•1980 / 8 = 247.5SlidesSlides•Hui Zhang, Randy Bryant, Dave EckhardtL08_IP15-441Computer Networking– 1 –15-441SynchronizationTextbookTextbook•Start reading Chapter 4 (mammoth!)•Today•“Internetworking”•Section 4.1, plus some, minus some•Upcoming•Routing (3 lectures): Section 4.2 (more or less)The other kind of learningThe other kind of learning•Which coding standard did you choose?•BSD/Linux/PDL?•Who has used source control?•Consider scheduled P2 meetings with your partner•You will be responsible reading for your partner's code...– 1 –15-441PreludeWhat does “TCP” stand for?What does “TCP” stand for?•What does it do?What does “IP” stand for?What does “IP” stand for?•What does it do that TCP doesn't???– 1 –15-441“Internetworking”??How can we study Internetworking: only one Internet?How can we study Internetworking: only one Internet?•Lots of world-spanning digital networks•Telegraph, telephone•IBM VNET•DEC DECNET•X.25 public data network (Europe)•IBM SNA•Xerox VinesWhat makes “The” Internet special?What makes “The” Internet special?– 1 –15-441“The Internet Way”What makes “The Internet” special?What makes “The Internet” special?•(Eckhardt's opinion)1. Heterogeneity1. Heterogeneity•The “Internet Problem” (Cerf & Kahn): three networks•Land-based computer network (ARPANET, Internet)•Mobile packet radio network•Satellite radio network•Each network “had its own ideas”•Node address, packet size range, medium access control, ...•Each network carefully designed to do “its thing” well»Environments very different, solutions very different»No way to declare “one way to do things”– 1 –15-441“The Internet Way”2. TCP 2. TCP IP IP•This decision was not immediately obvious•ARPANET “NCP” was one protocol•X.25 was one protocol•Everybody liked & understood reliable stream protocols•In some ways this decision was bad...•...(forward ref to congestion control lecture)...•...“See, it really wasn't obvious!”– 1 –15-441“The Internet Way”3. Semi-accidental “open standards” approach3. Semi-accidental “open standards” approach•“Supercomputers” were very different•From the beginning, lots of implementations of protocols•Cooperative/competitive interoperability “bake-off” events•...paid for by the (sole-source) funding agency.•Result: “of course” protocol docs were widely available•Versus»ISO, IEEE: development supported by people paying for access to standards documents– 1 –15-441“The Internet Way”4. The “RFC approach”4. The “RFC approach”•They're not really “standards documents”•More like “Hey, look what I did!”•As opposed to: “Observe my cool design for the future...”•“IETF credo” (Dave Clark, 1992)•We reject kings, presidents, and voting.•We believe in rough consensus and running code.ResultsResults•Not just one network technology•Not just one vendor's boxes, OS, database, ... (cf. IBM SNA)•Room in the protocols for innovation•Room in the culture for innovation– 1 –15-441What is an Internetwork?Multiple incompatible LANs can be physically connected by Multiple incompatible LANs can be physically connected by specialized computers called specialized computers called routersrouters..The connected networks are called an The connected networks are called an internetworkinternetwork..•The “Internet” is one (very big & successful) example of an internetworkhosthost hostLAN 1...hosthost hostLAN 2...router router routerWAN WANLAN 1 and LAN 2 might be completely different, totally incompatible LANs (e.g., Ethernet and ATM)– 1 –15-441Issues in Designing an InternetworkHow do I designate a distant host?How do I designate a distant host?•Addressing / namingHow do I send information to a distant host?How do I send information to a distant host?•“Service model”•What gets sent?•How fast will it go?•How often will it get there? Or else what?•Routing – which path(s) will my information take?ChallengesChallenges•Heterogeneity•Assembly from variety of different networks•Scalability•Ensure ability to grow to worldwide scale– 1 –15-441Internet Protocol (IP)““Hour-glass” ModelHour-glass” Model•Abstraction layer hides underlying technology from network application software•Make “as minimal as possible”•Allow range of current & future technologies•Can support many different types of applicationsNetwork technologyNetwork applications email WWW phone...SMTP HTTP RTP...TCP UDP…IP Ethernet PPPCSMA async SONET copper fiber radio...Steve Deering, CISCO– 1 –15-441Agreeing, DisagreeingHow to address a distant hostHow to address a distant host•Or else we'll never get information there•Ignore/work-around/use addressing method of local netHow to “containerize” dataHow to “containerize” data•What goes in a link-layer “frame”?•Potentially many kinds of data (mux/de-mux)•Format of key control items (sender, receiver)•Managing the size issueWe do We do notnot agree on agree on•Routing – Ethernet van-mounted packet radios•Precise service model – Token ring has link-level ACK– 1 –15-441IP Service Model•Low-level communication model provided by InternetDatagramDatagram•Each packet self-contained•All information needed to get to destination•No advance setup or connection maintenance•Analogous to letter or telegram0 4 8 12 16 19 24 28 31ver-sionHLenTOS LengthIdentFlagsOffsetTTL Protocol ChecksumSource AddressDestination AddressOptions (if any)DataHeaderIPv4 PacketFormat– 1 –15-441IPv4 Header Fields: Word 1Version: IP VersionVersion: IP Version•4 for IPv4HLen: Header LengthHLen: Header Length•32-bit words (typically 5)TOS: Type of ServiceTOS: Type of Service•Priority informationLength: Packet LengthLength: Packet Length•Bytes (including header)•Header format can change with versions•First byte identifies version•Length field limits packets to 65,535 bytes•In practice,
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