Transport Layer

What are the two main functions of the Transport Layer?

1) Delivers segments/datagrams via the Network Layer 2) Passes received payloads to the correct application using port numbers.

What range defines well-known ports?

0-1023 (e.g., HTTP=80, HTTPS=443), maintained by IANA.

How does TCP establish a connection?

Three-way handshake: SYN → SYN-ACK → ACK with random sequence numbers for synchronization.

What four reliability mechanisms does TCP provide?

1) Acknowledgments 2) Retransmission timeouts 3) Flow control (sliding window) 4) Congestion control.

What are key characteristics of UDP?

Connectionless, no retransmissions/flow control, optional checksum, low overhead, supports multicast.

How does TCP flow control work?

Receiver advertises available buffer space via sliding window; sender limits data to this window size.

What is the purpose of TCP's congestion window?

Limits sender’s transmission rate based on network capacity, starting with slow start (doubling per RTT).

Why might video streaming prefer UDP over TCP?

UDP avoids buffering delays from TCP’s retransmissions, allowing glitches instead of freezes (better for live streams).

Why does DNS primarily use UDP?

Lightweight queries/responses fit UDP’s simplicity; TCP is only used for large responses (>512 bytes).

What newer protocol uses UDP for HTTP/2?

QUIC, designed to reduce latency while maintaining reliability.

How does TCP handle packet loss?

Retransmits after timeout or duplicate ACKs, then reduces congestion window to mitigate network strain.

What identifies a unique communication endpoint?

Socket: IP address + port + protocol (TCP/UDP) – e.g., (192.168.1.1:80, TCP).

What happens when a TCP receiver advertises a window size of 0?

Sender pauses transmission but may send 1-byte probes to check for reopened window.

Compare TCP and UDP header overhead.

TCP: 20+ bytes (seq/ack numbers, flags, windows). UDP: 8 bytes (ports, length, checksum).

When might an application use both TCP and UDP?

Video conferencing: TCP for signaling/control, UDP for real-time audio/video streams.

What transport protocol is inherent to HTTP/3?

QUIC, running over UDP to combine reliability with reduced handshake latency.