COMT 6251 Network Layers COMT 625

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

COMT 6253 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 6254 An Example “Backbone” Web Server: Web Browser

COMT 6255 The Request User types into a web browser: “

COMT 6256 Browser Interpretation “ Protocol to use is HTTP (HyperText Transfer Protocol); the transport protocol is TCP The requested server is named The Service Access Point (SAP)is 80 The requested file is “index.htm”

COMT 6257 Name Resolution needs name to address mapping Make a call to the “resolver” (or, in most PCs, the “stub resolver”) to get a numeric address.

COMT 6258 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 6259 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 Request to TCP Browser makes a call to TCP, asking to open a connection to the numeric browser address

COMT 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 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 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 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 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 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 Receipt 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.

COMT 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 Standards Setting Internet Operations and Management Routing SecurityTransport User ServicesGeneralApplications Areas Working Groups Area Directors IETF ISOC IABIESG IANA

COMT The Domain Name System

COMT Rationale People remember names better that numbers (think 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 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 points to a host csm.ohiou.edu is a domain

COMT 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. EDUCOM ohiou.eduosu.edu csm.ohiou.edu cs.ohiou.edu

COMT 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. EDUCOM ohiou.eduosu.edu cs.ohiou.edu csm.ohiou.edu

COMT 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 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 Adminstration Allocation of IP addresses –Address availability Allocation of Domain Names –Trademark issues

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