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Transcript
Network Layers COMT 625 COMT 625 1 Overview • • • • IP and general Internet Operations Address Mapping ATM LANs Other network protocols COMT 625 2 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 COMT 625 3 An Example Web Server: www.lerc.nasa.gov Web Browser “Backbone” COMT 625 4 The Request • User types into a web browser: • “http://www.lerc.nasa.gov/index.htm” COMT 625 5 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” COMT 625 6 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. COMT 625 7 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. COMT 625 8 HTTP Message • The browser assembles a text message: • First line: –GET index.htm • Optional additional lines: –Browser type –HTTP version number –Transmission option negotiation COMT 625 9 Request to TCP • Browser makes a call to TCP, asking to open a connection to the numeric browser address COMT 625 10 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 COMT 625 11 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. COMT 625 12 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 COMT 625 13 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. COMT 625 14 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. COMT 625 15 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. COMT 625 16 Receipt • www.lerc.nasa.gov receives the packet. • IP removes its headers after errorchecking the header • TCP removes its headers, after errorchecking the entire packet. • TCP consults the owner of SAP 80, and determines that the web server is willing to create the connection. COMT 625 17 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. COMT 625 18 Standards Setting ISOC IANA IESG IAB IETF Area Directors Areas Internet Routing Operations and Management General Security User Services Applications Transport Working Groups COMT 625 19 The Domain Name System COMT 625 20 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 COMT 625 21 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 COMT 625 22 Naming Hierarchy • The Root Domain, “.” • International Top Level Domains (TLDs) . EDU COM –ISO Country Codes, .INT • Generic TLDs ohiou.edu osu.edu –.COM, .NET, .ORG, .GOV, csm.ohiou.edu .MIL, .EDU • Special Purpose - .ARPA COMT 625 cs.ohiou.edu 23 Zones of Authority • A Name-Server is said to be “Authoritative” for the domains it has actual database records for • A Name Server can service more than one domain. • Name Servers must be redundant . EDU ohiou.edu COM osu.edu csm.ohiou.edu cs.ohiou.edu COMT 625 24 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. COMT 625 25 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). COMT 625 26 Adminstration • Allocation of IP addresses – Address availability • Allocation of Domain Names – Trademark issues COMT 625 27 Other Protocols • Appletalk – Used in the Mac OS – Routable • NETBIOS – Windows operating syste,s – uses NETBEUI (non-routable) or – TCP/IP – Incorporates name mapping COMT 625 28