Application Layer Protocols: DNS, SMTP, HTTP, FTP & Email — Practice

Question 1

Which transport protocol and port does DNS use, and when does it switch?

Accepted Answer

UDP on port 53, switching to TCP for responses too large for a single datagram and for zone transfers.

Question 2

What does a 304 status code mean and what triggers it?

Accepted Answer

Not Modified, sent in response to a conditional GET carrying If-Modified-Since when the resource has not changed since that date. The response has no body.

Question 3

Name the two ports FTP uses and what each carries.

Accepted Answer

Port 21 carries the control connection, which lasts the whole session, and port 20 carries the data connection in active mode, opened per transfer.

Question 4

Count the messages to resolve a four-level name from a cold cache with iterative queries between servers.

Accepted Answer

One recursive query to the resolver, then two messages each for the root, TLD and intermediate referrals plus the authoritative answer, which is eight, then one reply to the host, giving ten in total.

Question 5

A page has a base file and 8 objects on one server with 100 ms RTT. Compare non-persistent, persistent, and persistent with pipelining.

Accepted Answer

Non-persistent costs two round trips per object for nine objects, which is 18 round trips or 1800 ms. Persistent costs two for the first and one for each of the remaining eight, which is ten round trips or 1000 ms. Pipelining collapses the eight into one, giving three round trips or 300 ms.

Question 6

Explain how a cookie mechanism works across three requests.

Accepted Answer

The first response carries Set-Cookie with a server-generated identifier. The browser stores it and returns it in a Cookie header on the second and third requests. The server keeps the actual session data in its own store keyed by that identifier, so the protocol itself remains stateless.

Question 7

Compare POP3 and IMAP on where messages live and what happens with multiple devices.

Accepted Answer

POP3 downloads messages and typically deletes them from the server, so messages live wherever they were downloaded and other devices never see them. IMAP keeps messages and folder structure on the server, synchronising state so every device sees the same mailbox.

Question 8

Explain why active-mode FTP fails behind NAT and how passive mode fixes it.

Accepted Answer

In active mode the client names a port and the server opens the data connection inbound to it, but NAT has no table entry for a connection the inside host did not initiate, so it is dropped. Passive mode has the server listen and the client connect outbound, which creates the NAT entry naturally.

Question 9

Why can email not be delivered end to end with SMTP alone?

Accepted Answer

SMTP is a push protocol requiring the receiver to be always on at a stable, reachable address, which holds for mail servers but not for a laptop that may be off, asleep or behind NAT. The message therefore stops at the recipient's server and must be pulled from there at an arbitrary later time, which is a different interaction needing its own protocol.

Question 10

State what HTTP/2 fixed, what it did not, and how HTTP/3 completes the job.

Accepted Answer

HTTP/2 multiplexes many streams over one connection and compresses headers, removing application-level head-of-line blocking where a slow response blocked those behind it. TCP's head-of-line blocking remains, since one lost segment stalls every stream sharing the connection. HTTP/3 runs over QUIC on UDP, where streams are ordered independently, and folds the cryptographic handshake into the transport handshake.

Practice — Application Layer Protocols: DNS, SMTP, HTTP, FTP & Email

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