COMT 625 1

Network Layers

COMT 625

COMT 625 2

Overview

• IP and general Internet Operations

• Address Mapping

• ATM LANs

• Other network protocols

COMT 625 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

COMT 625 4

An Example

“Backbone”

Web Server:

www.lerc.nasa.gov

Web Browser

COMT 625 5

The Request

• User types into a web browser:

• “http://www.lerc.nasa.gov/index.htm”

COMT 625 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”

COMT 625 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.

COMT 625 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.

COMT 625 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

COMT 625 10

Request to TCP

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

COMT 625 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

COMT 625 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.

COMT 625 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

COMT 625 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.

COMT 625 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.

COMT 625 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.

COMT 625 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.

COMT 625 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.

COMT 625 19

Standards Setting

Internet

Operations and Management

Routing

Security Transport

User ServicesGeneral ApplicationsAreas

Working Groups

Area Directors

IETF

ISOC

IABIESG

IANA

COMT 625 20

The Domain Name System

COMT 625 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

COMT 625 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

COMT 625 23

Naming Hierarchy

• The Root Domain, “.”

• International Top Level Domains (TLDs)–ISO Country Codes, .INT

• Generic TLDs–.COM, .NET, .ORG, .GOV, .MI

L, .EDU

• Special Purpose - .ARPA

.

EDU COM

ohiou.edu osu.edu

csm.ohiou.edu

cs.ohiou.edu

COMT 625 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

cs.ohiou.edu

csm.ohiou.edu

COMT 625 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.

COMT 625 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).

COMT 625 27

Adminstration

• Allocation of IP addresses– Address availability

• Allocation of Domain Names– Trademark issues

COMT 625 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