In networking, each layer adds a wrapper around the data with more information about data integrity or routing or another information.
At the highest level, you have your data that you want to transmit. As it is prepared for transmission on the network, the data is encapsulated in other data to help it arrive at its destination. Any particular piece of data will be wrapped, partially unwrapped, and re-wrapped as it moves from wire to wire across the Internet to its destination.
The act of wrapping data puts a new header on the data. This header encapsulates the original data, and all the headers that have been added before it.
Here is an example of a fully-encapsulated HTTP data packet.
+-----------------+
| Ethernet Header | Deals with routing on the LAN
+-----------------+
| IP Header | Deals with routing on the Internet
+-----------------+
| TCP Header | Deals with data integrity
+-----------------+
| HTTP Header | Deals with web data
+-----------------+
| Your Data | Whatever you need to send
| |
| |
+-----------------+
As the data leaves your LAN and heads out in the world, the Ethernet header will be stripped off, the IP header will be examined to see how the data should be router, and another header for potentially a different protocol will be put on to send the traffic over DSL, a cable modem, or fiber.
The Ethernet header is created and managed by the network drivers in the OS.
Under Unix-like operating systems, the sockets API is the one used to send Internet traffic. It supports both the TCP and UDP protocols, and IPv4 and IPv6.
The sockets API gives access to the IP and TCP layers in the diagram above.
A socket descriptor is a number used by the OS to keep track of open connections.
You can create a new socket (socket descriptor) with the socket() system call.
Once created you still have to bind it to a particular IP address (which the
OS associates with a particular network card). This is done with the bind()
system call.
Once bound, you can read and write data to the socket using the recv() and
send() system calls.
- See also Beej's Guide to Network Programming
The final piece of information needed for web traffic is the HyperText
Transport Protocol (HTTP). While TCP deals with general data integrity and IP
deals with routine, HTTP is concerned with GET and POST requests of web
data.
Like the other stages of networking, HTTP adds a header before the data it wants to send with the packet. Like IP and TCP, This header has a well-defined specification for exactly what needs to be sent.
Though the specification is complex, a fortunately small amount of information is needed to implement a barebones version.
For each HTTP request from a client, the server, sends back an HTTP response.
Here is an example HTTP GET request and response using version 1.1 of the HTTP
protocol getting the page http://lambdaschool.com/example:
GET /example HTTP/1.1
Host: lambdaschool.com
And here is a sample HTTP response:
HTTP/1.1 200 OK
Date: Wed Dec 20 13:05:11 PST 2017
Connection: close
Content-Length: 41749
Content-Type: text/html
<!DOCTYPE html><html><head><title>Lambda School ...
The end of the header on both the request and response is marked by a blank line (i.e. two newlines in a row).
If the file is not found, a 404 response is generated and returned by the server:
HTTP/1.1 404 NOT FOUND
Date: Wed Dec 20 13:05:11 PST 2017
Connection: close
Content-Length: 13
Content-Type: text/plain
404 Not Found
Important things to note:
- For HTTP/1.1, the request must include the
Hostheader. - The second word of the first line of the response gives you a success or failure indicator.
Content-Lengthgives the length of the request or response body, not counting the blank line between the header and the body.Content-Typegives you the MIME type of the content in the body. This is how your web browser knows to display a page as plain text, as HTML, as a GIF image, or anything else. They all have their own MIME types.- Even if your request has no body, a blank line still must appear after the header.
A web server is a piece of software that accepts HTTP requests (commonly GET requests for HTML pages), and returns responses (commonly HTML pages). Other common uses are GET requests for images within web pages, and POST requests to upload data to the server (e.g. a form submission or file upload).
We will write a simple web server that returns data on three GET endpoints:
http://localhost:3490/should contain some HTML, e.g.<h1>Hello, world!</h1>.http://localhost:3490/d20should return a random number between 1 and 20 inclusive astext/plaindata.http://localhost:3490/dateshould print the current date and time in GMT astext/plaindata.
Examine the skeleton source code for which pieces you'll need to implement.
For the portions that are already written, study the well-commented code to see how it works.
- Handle a
POSTrequest that sends a payload of plain text data to the/saveendpoint.- The web server should save this data in a file.
- The response should be of type
application/jsonand should be{"status":"ok"}. - What happens if multiple processes try to write to this file at the same time? How can you mitigate concurrency problems?
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