Web Protocols #general

Client #client

# Clients

Clients are computer programs that connect to servers

initiate a connection

Any computer can be a client

HTTP #http

# hypertext transfer protocol

how web servers and web browsers communicate

# HTTP verbs

HTTP requests begin with a VERB.
here are somethings each VERB is used for:

* GET - fetch a document
* POST - submit a form / send data / or sometimes upload a file
* HEAD - fetch metadata about a document
* PUT - upload a file
* DELETE
* OPTIONS

# HTTP headers

Headers have a key followed by a colon followed by a value:

key: value

# Curl commands

curl -s <url>  : get file from the url
curl -H        : set http request header
curl -sI <url> : only get the http response header
curl -d        : data
curl -X POST <url> -d ... : post data

Peer to peer #server

Aside from servers and clients, there is a thrid role in computer networks: peer.

In a peer to peer network, clients establish connections directly to other clients.
Nodes in the network are symmetric with no fixed central servers (both are clients and servers at the same time)

Examples of peer to peer protocols:

* bittorrent
* webrtc

Ports #ports

Each computer can have many services

A port is a number between 1 and 65535 that differentiates among the services on a system

# customary ports

* 21 - ftp (control port)
* 22 - ssh
* 25 - smtp
* 80 - http
* 443 - https
* 3306 - mysql
* 5432 - postgresql
* 5984 - couchdb
* 6667 - irc

# port and permissions

by default, systems can only listen to ports below 1024 as the root user

Servers #server

Sometimes when people say 'server' they mean a computer program that listens for incoming connections

Other times when people say 'server' they mean a computer that is configured to run server programs

Any computer that has a networking stack can be a server

SSH

# SSH is a binary protocol so we can't just 

web_protocols #protocols

Example of network protocols

# HTTP: browse web pages
# HTTPS: browse web pages with encryption
# SMTP: sends and receives emails
# IMAP, POP3: loads emails from an inbox
# IRC: chat
# FTP: file transfer
# SSH: remote shell over an encrypted connection
# SSL: low-level secure data transfer (used by HTTPS)