SIP Train Blog — protocol depth, in writing

SIP protocol

The SIP INVITE Lifecycle: 100 Trying through 200 OK ACK, with the retransmission rules that actually matter

27 April 2026 · 12 min read

The INVITE state machine, the timer table you actually need to memorise, and the difference between 2xx ACK and non-2xx ACK that catches engineers out. RFC-grade depth on the part of SIP everyone thinks they know.

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Media transport

RTCP in Practice: What Sender and Receiver Reports actually tell you

20 April 2026 · 10 min read

Reading SR/RR fluently — jitter in RTP units, RTT from LSR/DLSR, and what to do when your trace shows two RTCP packets in a 30-second call.

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Protocol depth, in writing.

The SIP INVITE Lifecycle: 100 Trying through 200 OK ACK, with the retransmission rules that actually matter

RTCP in Practice: What Sender and Receiver Reports actually tell you