TCP — Reliability, Congestion Control, and SIP over TCP/TLS

TCP's Contract: Reliable, Ordered, Byte-Stream Delivery

TCP guarantees that every byte sent arrives at the destination, in order, without corruption. This is a powerful contract — applications can write data to a TCP socket and trust that the other side receives it exactly as sent. But this contract comes with costs that matter enormously for real-time communications.

The Three-Way Handshake

Every TCP connection begins with a handshake:

Client → Server:  SYN         (seq=100)
Server → Client:  SYN-ACK     (seq=300, ack=101)
Client → Server:  ACK          (seq=101, ack=301)

SYN: "I want to connect. My initial sequence number is 100." SYN-ACK: "Acknowledged. My initial sequence number is 300." ACK: "Got it. We're connected."

This takes one full round-trip time (RTT) before any data flows. For a connection between New York and London (~70ms RTT), that's 70ms of setup delay. TCP with TLS (Lesson 17) adds more round trips.

For SIP over TCP, this means the first SIP message to a new server takes longer than SIP over UDP. However, once the connection is established, subsequent messages on the same connection have no setup delay.

💡 NOC Tip: en you see SIP over TCP connection failures, check for SYN packets without SYN-ACK responses in packet captures. A SYN with no reply means either the server isn't listening, a firewall is blocking, or the server is overloaded and its SYN queue is full. The client will retry with exponential backoff, causing visible call setup delays.

Sequence Numbers and Acknowledgments

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