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Web

This page contains information about the web part of the course.

This should be filled with information, But it is not right now. You can submit a PR to fix this!

lecture 1:

  • 20% lab
  • 40% web
  • 40% database

organisations:

W3C --> negotiator between browsers so implementations are standardised between browsers IETF --> Request for Comment

HTTP

HTTP/1.1, RFC 2086

more support

server waits client initiates conversation (request) server handles requests (response)

HTTP/2, RFC 7540

less support

Content type

MIME:

pattern: [primary object type]/[subtype] examples:

  • text/plain
  • text/html
  • image/jpg
  • unknown/unknown
  • application/xml
  • model/mesh

Content length

if size of body doesnt match header length --> reject

also important for persistent connections. if multiple http requests are sent this field can help the browser find the end of the stream of requests

Content encoding

tells what kind of compression the server or client understands and thus can send to eachother.

advantage: less bandwidth disadvantage: more cpu time

examples:

  • gzip
  • deflate
  • identity

Content MD5

removed in HTTP 2.0 though it's still used. used to verify content.

md5 is a hash computed of the entire body. if a bit is changed the md5 is different. so if the client recalculates md5 and it doesnt match the server knows there is something wrong

Expires

lets the client know when the content is not valid anymore and the client has to re-request it. this however has in history rarely actually been used by clients and is mostly used by webcaches. when a webcache has a recource, and it is not expired it will serve it, else it is forwarded to the origin server (Eventually, it might go through multiple webcaches)

web browsers also cache webpages. Therefore nowadays the expires field is more useful for clients too.

Last modified

again mostly used by webcaches to know when it has to re-request data to cache. used to be used by search engines to give higher rankings to recently modified pages. due to abuse searchengines dont do this anymore.

Connection & Upgrade

Used so the server can push to clients (instead of client initiating requests)

simulating server --> client requests:

  • polling
  • long polling (server keeps conenction open until new data arrived)

HTTP/1.1 doesnt allow more than this

Websocket protocol:

enables bidirectional communcation between client and server

client initiates bidirectional communication client sends 2 headers:

if server allows connection upgrade to websockets then bidirectional communication is established

Status code

1xx informational

2xx request ok

3xx redirected, resource moved

4xx client error 404 resource not found

402: reserved for payment requred

5xx server error should never be seen. server errors shouldnt be seen by clients!

HTTP Methods

GET sends resource, header and body

HEAD only send the header, no body

POST WIP

PUT WIP

TRACE WIP

OPTIONS WIP

DELETE WIP

IP addresses

DNS server converts domain name to ip address

ipv4:

xx.xx.xx.xx (hex numbers) max 2^32 ip adresses push to convert to ipv6

ipv6:

xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx (hex numbers) 2^128 possibilities