https://siptrain.com/blog/ Deep technical writing on SIP, RTP, SBCs, SS7/SIGTRAN, and carrier voice engineering. en https://siptrain.com/blog/one-way-audio-diagnosis-tree https://siptrain.com/blog/one-way-audio-diagnosis-tree Mon, 08 Jun 2026 09:00:00 +0000 Six root causes of one-way audio with a PCAP fingerprint for each, and a four-step decision tree that resolves the overwhelming majority of one-way audio tickets in under five minutes. https://siptrain.com/blog/sigtran-sctp-m2pa-m2ua-m3ua https://siptrain.com/blog/sigtran-sctp-m2pa-m2ua-m3ua Mon, 01 Jun 2026 09:00:00 +0000 SCTP, M2PA, M2UA, and M3UA explained: the layer each terminates, the ASP state machine, and the failure signatures every interconnect engineer should know on sight. https://siptrain.com/blog/sip-register-contact-expiry-nat-keepalives https://siptrain.com/blog/sip-register-contact-expiry-nat-keepalives Mon, 25 May 2026 09:00:00 +0000 What the registrar actually stores, how Contact lifetime is negotiated, the five trace fingerprints of broken registration, and the keep-alive arithmetic for UDP, TCP, and TLS behind real-world NATs. https://siptrain.com/blog/sip-transport-fragmentation-1300-byte-rule https://siptrain.com/blog/sip-transport-fragmentation-1300-byte-rule Mon, 18 May 2026 09:00:00 +0000 Why RFC 3261's 1300-byte transport-switch rule exists, how a fragmented INVITE actually fails in production, and what to migrate to when STIR/SHAKEN starts pushing message sizes past the line. https://siptrain.com/blog/isup-cause-codes-to-sip-rfc3398 https://siptrain.com/blog/isup-cause-codes-to-sip-rfc3398 Mon, 11 May 2026 09:00:00 +0000 Q.850 to SIP mapping per RFC 3398, the lossy collapse points, and how the Reason header (RFC 3326) keeps the original Q.850 cause visible end-to-end through a carrier interconnect. https://siptrain.com/blog/top-10-sip-failure-modes https://siptrain.com/blog/top-10-sip-failure-modes Mon, 04 May 2026 09:00:00 +0000 Ten failure modes that account for the majority of production SIP tickets, the precise trace fingerprint each leaves, and the RFC clause that explains the behaviour. https://siptrain.com/blog/sbc-topology-hiding https://siptrain.com/blog/sbc-topology-hiding Mon, 27 Apr 2026 09:00:00 +0000 Every header and SDP field a B2BUA SBC rewrites between the access and core legs, plus the three pathologies that show up in paired traces. https://siptrain.com/blog/sip-invite-lifecycle https://siptrain.com/blog/sip-invite-lifecycle Mon, 27 Apr 2026 09:00:00 +0000 The INVITE state machine, the retransmission timers you actually need to memorise, and the 2xx-vs-non-2xx ACK routing distinction. https://siptrain.com/blog/rtcp-in-practice https://siptrain.com/blog/rtcp-in-practice Mon, 20 Apr 2026 09:00:00 +0000 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.