Http Basics

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HTTP Basics

The Original Slide deck on which this half formed page is based can be found here.

Background

There are many devices in our lives that host or serve content:

The TCP/IP protocol is used to bind these diverse tools togother:: In a sense, TCP/IP is the Internet!

History

The Internet was created at the instigation of a government organization called DARPA Defense Advanced Research Projects Agency. It is a relatively small branch of the Department of Defense (DoD) with 240 employees, half technical. It's job is to create new technologies for the military.

The first important network they created was called the ARPANET: the  Advanced Research Projects Agency Network. It was a packet switching network, but was originally based on NCP rather than TCP/IP. It was not until 1983 that the ARPANET adopted TCP/IP, but Vinton Cerf, the creator of NCP, began work on TCP/IP in 1973.

Funded by DARPA Created the Internet in late 1969

TCP/IP Created in early 70's, circa 1973. Primary Authors: Vinton Cerf and Bob Kahn where were looking for a way to connect ARPANET computers

TCP/IP

TCP/IP is The protocol of the Internet.

The name is created by combining to acronyms:

TCP describes how to make packets, while IP describes how to route them.

To find where a particular router is located, based on an IP address, go to:

Data Packets

Rather than transmitting an entire file at once, TCP/IP sends information across the web in packets. A file is broken up into packets and sent one packet at a time. As result, the intelligence is at the endpoints: first the place where the data is broken up into packets, and then on the other end where the data is reassembled from packets.

Of course, there is significan't technology involved in the transport layer, which handles the routing and ferries the packets from the origin to the source. One of the interesting things about TCP/IP is that packets can be routed differently. They are then assembled at the destination, regardless of what route they took, or in what order they arrived.

Each packet has a destination address stored in a header. Each machine that is sent a pack reads the destination address found in the hearder, and then passes it down the line.

LANs and WANs

A WAN typically encompasses a large area such as a city, county, state or country. A LAN, on the other hand, typically encompasses only a single home, or even a single room in a home. Sometimes a LAN will stretch over several buildings. A LAN is usually comprised of a single subnet.

Switches, Router and Gateways

Switches, routers and gateways are very similar tools. A switch or a hub can be thought of as a primitive router. A router is a small computer located between two networks. It forwards packets between the two networks by reading the address in a packet and looking up its destination in a table. A typical example would be a router that stands between one or more home networks and a Wide Area Network. If there is a need to link two networks that use different protocols, then the machine or tool used to bridge the networks is called a gateway. One can think of a router as being a simple gateway.

Most routers sold for the home market also contain tools like DHCP servers and firewalls.

HTTP Transactions

The transactions are stateless. Use GET, POST, PUT, DELETE, HEAD, TRACE and OPTIONS.

Every transaction has a header, which can be examined, and a body which need not be examined until it used by the client or server.

UDP

TCP guarantees delivery of information, but UDP just sends a packet and hopes for the best. This may sound rather polyannish, but sometimes UDP is a good thing. For instance, if you are streaming TV or music then it would be better to drop a little bit of data than to cancel the transmission altogether. When you are listening to music or watching videos, you will sometimes notice these flaws in the transmission.

URLS

A URL is a scheme for making an IP address human readable.

Here is hostname: www.bellevuecollege.edu

You can register a host name with company designated to guarentee that the names are unique.

It is important to understand the different parts of a URL.

URL Scams

IP Addresses

Subnets

The various sections of an IP addresses are broken up by periods, and these periods are used to define subnets. For instance, the subnet beginning with 192.168.1.X can consist of up to 256 nodes designated by the digits found in X. Consider the first set of numbers: 192. This number consists of 8 bits, which means if can have a value between 0 and 255. All four sets of numbers added together are 8 * 4 or 32 bits. If we write 192.168.1.0/24, then that means we consider the first 24 bits (192.168.1) to be the network prefix, and the last 8 bits are used to address a particular host. The values 255.255.255.0 is the network mask for routing prefix 192.168.1.0/24. We use a router to move data between different subnets.

HTTP SSH

Windows Tools

SMB

Server Message Block allows a machine to share services such as printers, files, named pipes and serial ports. In general, a client asks a server for these services using the SMB protocol, which frequently runs over TCP/IP. This is primarily a Microsoft Windows protocol, and to some degree has been replaced by Active Directory, though Windows machines still tend to run this protocol.

NETBIOS

NETBIOS is an old network protocol for sending messages between computers. Microsoft uses it heavily when setting up Local Area Networks. It now runs on top of TCP/IP port 139 so that it can be routed across a WAN. Originally, it did not run on TCP/IP and therefore could only be used on a LAN. NETBEUI extends NETBIOS by more rigorously defining the protocol for sending messages. There is a NETBIOS naming service that can be used to explicitly name a server. This is not technically the same thing as a Host Name, though the Host Name and the NETBIOS name can be the same, even though they are two separate entities. An example use for NETBIOS might be to request a file share.

NETSTAT

This is a utility that ships with Windows. It can tell you about the status of your network. Here, for instance, you can see what services are using your network:

netstat -ano

The ports are listed on the left. One of the columns you get back is PID. Use the Task Manager, on the Processes page, to translate a PID number to an application. You may need to use View | Select Columns to make the PID visible.

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