HTTP

uni
It stands for (HyperText Transfer Protocol).
This protocol is based on the Client-Server paradigm.
Two type of http messages:

• request
• response
  the message format is ASCII
  After a a timeout the connection is closed automatically
  TCP connection consume memory and leave them open leaves the data structures allocated waisting memory

Status Codes

there are multilple status codes for http responses

• 200 → Ok
  • request succeeded
• 301 → Move Permanently
  • requested object is moved, new location is specified later in the message with the tag Location
• 400 → Bad Request
  • request msg not understood by the server
• 404 → Not Found
  • requested document not found on this server
• 505 → HTTP version not supported

TELNET

telnet <host> <port> cli command open a tcp connection to the specified url and port
type GET <url> HTTP/1.1 to make a get request then press enter
type Host: <host> and press enter twice so have the response

Cookies

Used by some browers to maitain some state between transactions
Four Components:

• Cookie Header Line of http response message
• Cookie Header Line of http request message
• Cookie File kept on the user’s end system
• Back-End Database at the Web site

The client make a simple http request to a server
The server creates and ID for the user and store it in a database

This means that the next time you make a http request the server know who you are
This is how website can keep you logged in even when the website is closed

They have various implementations:

• Authorizations
• Shopping Carts
• Reccomendations
• User session state (web, e-mail)

The question is How do we keep the state?
protocol endpoints: maintain state at sender/reciever over multiple tansactions
cookies: HTTP messages carry state

The Cookies permits the sistes to learn about you on their site
Third party persistent cookie allow common identity to be tracked across multiple web sites

Web Caches (proxy servers)

Goal: satisfy client request without involving origin server
Why: to alleviate the load on origin servers. Caches are closer to clients so it reduces response time. Caches also alleviate the load on internet as a whole.
The user configures the browser to point to a web cache.

• if the objects is in the cache: the cache return object to client
• if the object isn’t in cache: the cache ask and receives the object from the origin server, caches it and sends it to client

Proxy servers act both as a client and as a server. They are typically installed by ISP.

Problem of recency of cached objects

What do we do if the object is updated on the remote servers and the proxy server isn’t aware?
Conditional GET:
goal: don’t send object if cache has up-to-date version

• Cache: the cache when sending the GET statement includes the version of the cached copy:
  if-modified-since:<date>
• Server: response contains no object if the cached copy is up to date:
  http/1.0 304 Not Modified

Important updates

HTTP 1.1

In this versione the scheduling algorithm for GET requests is FCFS: first come first served.
Issue: small objects may wait for bigger (slower) objects (requests) (HOL blocking: Head of line blocking). Also loss recovery (retransmitting lost TCP segments) stalls object transmission.

HTTP 2

RFC 7540, 2015: The key goal for this version was decreasing delay in multi-object HTTP requests.
Increased the flexibility for servers when sending objects to clients:
Objects are divided into frames and the frame transmission is interleaved.
Problems: recovery still stalls transmission, also no extra security over vanilla TCP connections.

HTTP 3

Adds

• security
• per object error (and congestion) control (more pipelining) over UDP