TCP Options

Question 1

What three parameters identify a socket option?

Answer 1

1. Option level (protocol), 2. Option name (e.g., SO_REUSEADDR), 3. Option value (integer/string/float).

Question 2

What is the purpose of SOL_SOCKET?

Answer 2

A socket level for protocol-independent options (e.g., SO_RCVBUF, SO_REUSEADDR).

Question 3

How do SO_RCVBUF and SO_SNDBUF differ from the recv() buffer?

Answer 3

SO_RCVBUF/SO_SNDBUF set OS-level buffer sizes for receiving/sending data, while recv() specifies max bytes to read per call.

Question 4

What does SO_REUSEADDR enable?

Answer 4

Allows multiple sockets to bind to the same address (useful for restarting servers or bypassing TIME_WAIT). Must be set before binding.

Question 5

How does SO_EXCLUSIVEADDRUSE conflict with SO_REUSEADDR?

Answer 5

SO_EXCLUSIVEADDRUSE prevents address sharing even if SO_REUSEADDR is set. Common on Windows for security.

Question 6

What does SO_ACCEPTCONN indicate?

Answer 6

Returns 1 if the socket is in listening mode (via listen()), 0 otherwise. Read-only.

Question 7

How does SO_LINGER affect socket closure?

Answer 7

Controls unsent data during close(). If enabled, blocks until data is sent or a timeout expires (configurable).

Question 8

What is the default value of SO_KEEPALIVE?

Answer 8

Disabled (0). Enables periodic connection checks to detect dead peers. Requires TCP (SOCK_STREAM).

Question 9

What additional TCP options refine SO_KEEPALIVE?

Answer 9

TCP_KEEPCNT (probe count), TCP_KEEPIDLE (idle time before probes), TCP_KEEPINTVL (interval between probes).

Question 10

What does SO_BROADCAST enable?

Answer 10

Allows UDP sockets to send messages to broadcast addresses (e.g., 255.255.255.255). Disabled by default.

Question 11

How do SO_RCVTIMEO and SO_SNDTIMEO differ from settimeout()?

Answer 11

They set separate timeouts for receive/send operations, while settimeout() applies to all blocking methods.

Question 12

What is TCP_FASTOPEN (TFO) used for?

Answer 12

Allows data exchange during TCP handshake (SYN packets). Requires a cookie setup. Disabled by default except on macOS.

Question 13

How does TCP_NODELAY affect data transmission?

Answer 13

Disables Nagle’s algorithm, reducing latency by sending small packets immediately instead of buffering.

Question 14

What is the syntax to set SO_REUSEADDR?

Answer 14

socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

Question 15

Why might setting SO_RCVBUF to 262,144 fail on Windows?

Answer 15

Windows limits buffer sizes (tested max is 262,144 bytes). OS may override larger values.

Question 16

What happens if SO_LINGER is set with a timeout?

Answer 16

The socket blocks during close() until data is sent or the timeout expires, then sends RST to force closure.

Question 17

What protocol family is required for SO_BROADCAST?

Answer 17

AF_INET (IPv4). Does not work with SOCK_STREAM or IPv6.

Question 18

How to check if a socket is in listening mode?

Answer 18

Use socket.getsockopt(SOL_SOCKET, SO_ACCEPTCONN). Returns 1 if listening.

Question 19

What is the default buffer size for SO_RCVBUF on Windows 11?

Answer 19

65,536 bytes (may vary by OS).

Question 20

Why use TCP_KEEPIDLE with SO_KEEPALIVE?

Answer 20

To configure idle time (default 7200s) before sending keepalive probes.

Question 21

What is the purpose of Nagle’s algorithm?

Answer 21

Reduces network overhead by buffering small packets into larger segments. Disabled via TCP_NODELAY.

Question 22

What are the two steps in TCP Fast Open (TFO)?

Answer 22

1. Client requests cookie via SYN. 2. Client sends data in SYN with cookie for future connections.

Question 23

How to enable UDP broadcast in Python?

Answer 23

Set SO_BROADCAST to 1: s.setsockopt(SOL_SOCKET, SO_BROADCAST, 1).

Question 24

What happens if SO_EXCLUSIVEADDRUSE is set on Windows?

Answer 24

Prevents other sockets from binding to the same address, even if they use SO_REUSEADDR.