Application layer

The Domain Name System maps host names such as www.example.com to IP addresses. Names are read right to left: the root (an empty label), then a top-level domain (TLD) such as com or in, then a second-level domain such as example, then host labels. No single server holds all names; the database is distributed over a tree of servers.

A zone is the part of the tree one organisation manages. The local resolver is configured on the host (or given by DHCP); the host sends every query to it. DNS messages normally travel over UDP, destination port 53. TCP port 53 is used when a response is too large for UDP or for zone transfers between servers.

In a recursive query the server asked takes over the job: it must return the final answer (or an error) and contacts other servers itself. In an iterative query the server asked returns either the answer or a referral (the name of a better server to ask), and the asker follows it. The usual setup is: the host sends a recursive query to its local resolver, and the resolver sends iterative queries to the root, the TLD and the authoritative server. Other combinations (for example fully recursive up the chain) are possible; read the question for which queries are recursive.

Caching: every server may keep records it has learnt. Each record carries a TTL (time to live, in seconds); the cache may use it only until the TTL expires, then it must discard it and ask again. Caching also keeps the referrals: the NS records of the com TLD servers are usually already cached, so the root is rarely contacted.

A DNS record (resource record) holds four things: name, value, type and TTL. The types a GATE candidate should know are below.

HTTP/1.0, HTTP/1.1 and HTTP/2 run over TCP (default port 80; port 443 for HTTPS, which is HTTP over TLS). HTTP/3 runs over QUIC, which uses UDP. The client sends a request and the server returns a response. HTTP is stateless: the server keeps no memory of earlier requests from the same client.

The first digit gives the class: 1xx informational, 2xx success, 3xx redirection, 4xx client error, 5xx server error. A conditional GET carries an If-Modified-Since header; if the cached copy is current the server answers 304 and sends no object.

Cookies let a stateless server recognise a user. The server's response carries a Set-Cookie header with an identifier; the browser stores it and returns it in a Cookie header on later requests to that site; the server keeps the details in a back-end database keyed by that identifier. The protocol stays stateless; the state lives in the cookie and the server's database.

A web cache (proxy server) sits between clients and origin servers. The browser sends its request to the cache; if the cache holds a fresh copy (a hit) it returns it, otherwise (a miss) it fetches the object from the origin server, stores a copy and returns it. The cache can check a stored copy with a conditional GET. A cache acts as a server to the client and as a client to the origin server.

A page is a base HTML file plus $n$ referenced objects (images and so on). The cost is counted in round-trip times (RTT). Convention used here: TCP setup takes 1 RTT; after it the request goes out and the first bytes of the response return in 1 RTT; connection teardown and TCP slow start are ignored; transmission time of the objects and DNS time are negligible unless the question gives them (if a transmission time is given, add it each time that object is sent); the browser learns of the objects only after the base file arrives.

HTTP/1.0 uses non-persistent connections by default: the server closes the TCP connection after sending one object. HTTP/1.1 uses persistent connections by default: several objects travel over one connection, and requests may be pipelined.

SMTP (TCP port 25) pushes mail from the sender's user agent to its mail server and from that server to the recipient's mail server. It is a push protocol, uses a persistent connection, and sends the message with a few commands (HELO, MAIL FROM, RCPT TO, DATA, QUIT). In classic SMTP the message body and headers are 7-bit ASCII text; other content is encoded (MIME). The recipient then pulls the mail from its own mail server with a mail access protocol.

FTP (File Transfer Protocol) uses two kinds of TCP connection. The control connection (server port 21) carries commands and replies, stays open for the whole session, and is out of band. Each file transfer or directory listing uses a new data connection, closed when that transfer ends. In active mode the server opens it from its port 20 to a port the client named; in passive mode the client opens it to a port the server named. Because the control connection stays open, FTP keeps state about the user during a session (the logged-in user, the current directory), unlike HTTP.