Application protocols

Vocabulary

English	Meaning
DNS/ˌdiː en ˈes/	The distributed domain name system used to look up records such as addresses for names.
HTTP/ˌeɪtʃ tiː tiː ˈpiː/	An application protocol using requests and responses to transfer representations of resources.

Resolve the name

DNS turns a name into an address by hierarchical delegation: the root knows the TLD servers, .com knows the registrars, the authoritative server knows the host. Each answer is a referral downward, and caching skips the walk for the answer's TTL.
HTTP is a request-response grammar: request line, headers, blank line, body — and the status codes class is the promise: 2xx success, 3xx redirect, 4xx client error, 5xx server error.

{exercise:uni-cs0905-dns}

Practice

DNS delegation means the root answers with —

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GET /index.html HTTP/1.1 with a Host header; the server answers 200 OK, headers (Content-Type, Content-Length: 128), blank line, exactly 128 body bytes.
The blank line is load-bearing: the parser waits for exactly those two newlines. Content-Length counts body bytes only — headers are counted by the blank line's position.

{exercise:uni-cs0905-bytes}

Practice

The header block ends at —

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200 (here is the body), 301 (it moved — Location says where), 404 (you asked wrong), 500 (I broke). A crawler treats 301 as a pointer, 404 as a dead end, 500 as retry-later.
One global name table would be one global outage: delegation splits the failure domain as well as the data.

{exercise:uni-cs0905-codes}

Practice

A 301 response tells the client —

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