N9. HTTP

Question 1

What does HTTP stand for?

Answer 1

Hyper Text Transfer Protocol

Question 2

What is HTTP?

Answer 2

• protocol for www
• client-server protocol
• application level protocol (runs over TCP)
• session: stateless (single req, single resp)
• text-based (printable strings for headers and content of messages)

(focus on HTTP/1.1)

Question 3

In an HTTP response header, what would A and B refer to for content-type: A/B

Answer 3

A is the type of media e.g. text
B is the actual content type of the payload e.g. HTML

Type A/subtype B
e.g. text/html

Question 4

What are the three common HTTP methods?

Answer 4

GET
- request specific page/object
- request has header only, no message body
HEAD
- request only header, not object itself
- can check timestamp against local cache
- request has header only, no message body
POST
- send form data to server
- request body contains form data

Question 5

What are the HTTP response code types? Give some examples

Answer 5

Types:
1xx Informational
2xx Successful
3xx Redirection 
4xx Client error
5xx Server error

Examples:
200 OK
301 Moved permanently 
404 Not found
502 Bad Gateway

Question 6

What does it mean for HTTP to be stateless?

Answer 6

• client requests treated independently
• server does not maintain state/history of previous client requests
  (notion of application-level session must be built into the app. using HTTP)
• client must fetch all page contents by sending separate GET requests for components of the page

Question 7

What is the main difference between HTTP 1.0 and HTTP 1.1 when fetching a page.

Answer 7

(A TCP connection must first be established)
HTTP 1.0 - closes TCP connection and creates a new one for every object linked on page
[Connection: close]
HTTP 1.1 - TCP connection kept open to fetch multiple objects
(reduced overhead)
[Connection: keep-alive]

Question 8

Properties of HTTP/2

Answer 8

• normal HTTP sitting over TLS
• multiplexing: one TCP connection for multiple requests
• piping: client can send requests in parallel and they are dealt with on one connection
• uses binary encoding for compression instead of text-based protocol
• server PUSH: server pre-emptively sends objects to client (caching)

Question 9

Properties of HTTP/3

Answer 9

• still being developed
• will run over QUIC (UDP) not TCP
• built-in security
• faster connection set-up
• features of HTTP/2

Question 10

What is the cache?

Answer 10

• performance optimisation tool
• localised copy of data
• means data is ‘closer’ to where it needs to be (spatial locality)
• copied to cache ahead of time (temporal locality)
• if original version of data changes, cached data will not (stale) because it is a copy

Question 11

What is a proxy?

Answer 11

Something (system or router) that takes the place of the client to communicate with the server

• for caching, the proxy can have a shared cache, allowing multiple clients to access cached resources from one place
• could be used for security/privacy to monitor what all clients accessing a site are viewing

Question 12

For caching, state some example header fields from requester and responder

Answer 12

Requester
- max-age 
- min-fresh
- max-stale
Responder
- must-revalidate 
- no-cache
- no-store
etc.