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Network Layers

Network Layers. COMT 625. Overview. IP and general Internet Operations Address Mapping ATM LANs Other network protocols. The Network Layer. Impose a global addressing scheme May also Provide virtual circuit connections Segment and re-assemble packets

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Network Layers

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  1. Network Layers COMT 625

  2. Overview • IP and general Internet Operations • Address Mapping • ATM LANs • Other network protocols

  3. The Network Layer • Impose a global addressing scheme • May also • Provide virtual circuit connections • Segment and re-assemble packets • Provide “user-friendly” name-based addressing

  4. An Example Web Server: www.lerc.nasa.gov Web Browser “Backbone”

  5. The Request • User types into a web browser: • “http://www.lerc.nasa.gov/index.htm”

  6. Browser Interpretation • “http://www.lerc.nasa.gov/index.htm” • Protocol to use is HTTP (HyperText Transfer Protocol); the transport protocol is TCP • The requested server is named www.lerc.nasa.gov • The Service Access Point (SAP)is 80 • The requested file is “index.htm”

  7. Name Resolution • www.lerc.nasa.gov needs name to address mapping • Make a call to the “resolver” (or, in most PCs, the “stub resolver”) to get a numeric address.

  8. Name Resolution cont. • The answer may come from local storage, or may itself require a network transmission using the Domain Name System (DNS) protocols. • The name space and the address space are unrelated, except that they can be translated into each other.

  9. HTTP Message • The browser assembles a text message: • First line: • GET index.htm • Optional additional lines: • Browser type • HTTP version number • Transmission option negotiation

  10. Request to TCP • Browser makes a call to TCP, asking to open a connection to the numeric browser address

  11. Request to IP • TCP creates a header with the origin and destination SAPs • TCP sets a “flag” (bit) to request a new connection (SYN) • TCP may set other parameters and options

  12. Locate the Gateway • IP needs to find a way to get the TCP PDU (called a segment) to the destination address. • First decision: is the (IP) address in the current broadcast domain? • Since in this case the destination is on a different network, it needs a gateway.

  13. Locate the Physical Gateway Address • IP knows the global (IP) address of the gateway from the local configuration storage. • To make a request of the data link layer, it needs the physical (MAC - Media Access Control) address of the gateway. • The same mapping would be needed if the target is on the same LAN

  14. Physical Gateway Address cont. • The ARP protocol is used to broadcast a request for the “owner” of the IP gateway address. • The gateway responds to the broadcast; IP learns the MAC address from the response.

  15. Dispatch the SYN Packet • IP creates one or more packets (usually one in this case). • The header has the origin and destination IP addresses and other options. • The packet goes to the Ethernet driver, which attaches the MAC origin address and the gateway MAC destination.

  16. Routing • The gateway receives the packet, and determines that it is not: • addressed to the gateway itself (at the IP level) • addressed to any networks the gateway is attached to • The gateway examines its routing table, and selects a circuit to send the packet out on.

  17. Receipt • www.lerc.nasa.gov receives the packet. • IP removes its headers after error-checking the header • TCP removes its headers, after error-checking the entire packet. • TCP consults the owner of SAP 80, and determines that the web server is willing to create the connection.

  18. Response • TCP acknowledges the receipt of the SYN (technically it could send data in this packet). • TCP responds to any options that may have been requested in the SYN • Browser actually transmits its request and the data flow starts.

  19. Standards Setting ISOC IANA IESG IAB IETF Area Directors Internet Routing General User Services Applications Areas Operations and Management Security Transport Working Groups

  20. The Domain Name System

  21. Rationale • People remember names better that numbers (think 1-800-flowers) • Names should be assigned and controlled within the organization that “owns” the named hosts • Names must be unique A Distributed Database is Required

  22. Name Structure • abc.xyc.foo.bar • Some portion (or all) of this name designates the “domain” • If the name designates a host, the leftmost portion of the name designates the host • www.ohiou.edu points to a host • csm.ohiou.edu is a domain

  23. Naming Hierarchy . • The Root Domain, “.” • International Top Level Domains (TLDs) • ISO Country Codes, .INT • Generic TLDs • .COM, .NET, .ORG, .GOV, .MIL, .EDU • Special Purpose - .ARPA EDU COM ohiou.edu osu.edu csm.ohiou.edu cs.ohiou.edu

  24. Zones of Authority • A Name-Server is said to be “Authoritative” for the domains it has actual data-base records for • A Name Server can service more than one domain. • Name Servers must be redundant . EDU COM ohiou.edu osu.edu csm.ohiou.edu cs.ohiou.edu

  25. Query Structure • The Domain Name system is designed to hold multiple pieces of information under any given name; a request must designate the Query Type • Technically, information for multiple protocol stacks can be stored; the normal Query Class is “Internet”. • A query can request that the contacted name server complete a query, even if it has to contact other Name Servers – a Recursive query • A Non-Recursive, or Iterative query will provide the address of the next name server instead.

  26. Query Process • A “Resolver” must know at least on Name Server. • A Name Server must at least know one Root Name Server. • The Resolver formulates the query and contacts its know Name Server. • Name Servers store, or cache, answers to recent queries, and supply those when the same question is asked again (e.g., by another host).

  27. Adminstration • Allocation of IP addresses • Address availability • Allocation of Domain Names • Trademark issues

  28. Other Protocols • Appletalk • Used in the Mac OS • Routable • NETBIOS • Windows operating syste,s • uses NETBEUI (non-routable) or • TCP/IP • Incorporates name mapping

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