Layer 4 picks one of two transport protocols. TCP is reliable and ordered; UDP is fast and lossy. This chapter explains the three-way handshake, sliding windows, port numbers, and the table of well-known ports worth knowing cold.
TCP guarantees that bytes arrive in order with no losses. It does this with a connection setup (SYN, SYN-ACK, ACK), per-segment sequence numbers, acknowledgments, retransmissions, and a sliding window for flow control.
Cost: latency (the handshake adds a round trip) and overhead (20+ bytes of header per segment, plus state on both endpoints).
Client ──SYN──────────▶ Server
Client ◀──SYN, ACK─── Server
Client ──ACK──────────▶ Server
(connection established — data flows)UDP just sends datagrams. No handshake, no retransmissions, no ordering guarantees. The header is only 8 bytes. If a datagram is lost, the application either accepts the loss or rebuilds reliability on top.
When you choose UDP: real-time voice and video (a delayed packet is worse than a missing one), DNS queries (one tiny request, one tiny reply), and DHCP (the client doesn't even have an IP yet).
2 more sections in this chapter, plus a 4-question quiz.
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