SIP Train Blog
Long-form technical writing on the topics SIP Train teaches. Aimed at working engineers, written from the trace up.
NAT pinhole asymmetry, SDP-vs-reality, SRTP keying asymmetry, asymmetric routing, firewall ACL drops, and codec/PT/transcoding asymmetry - each with a Wireshark fingerprint and the fix. Plus a four-step decision tree you can pin above the desk.
Read the post →SCTP's multi-streaming, multi-homing, and PPID, the differences between M2PA/M2UA/M3UA and where each one is used, and the ASP state machine plus DUNA/DAVA/DUPU/SCON messages that govern every modern SS7-over-IP interconnect.
Read the post →Per-Contact expiry, RFC 5626 outbound, Path vs Service-Route, ghost bindings under NAT rebinding, the 401 nextnonce cost, and the keep-alive arithmetic that decides whether your fleet survives the next refresh storm.
Read the post →RFC 3261's 1300-byte transport-switch rule, why fragmented INVITEs vanish silently in production, the trace fingerprint of a fragmentation failure, and how to migrate UDP-to-TCP without breaking Record-Route or losing connection reuse.
Read the post →Q.850 cause classes, the canonical ISUP-to-SIP mapping per RFC 3398, and how the Reason header (RFC 3326) keeps the original cause visible end-to-end when a 503 would otherwise collapse the whole class-2 range.
Read the post →Ten failure modes that account for the majority of production SIP tickets: DNS NAPTR/SRV failure, INVITE retransmission storms, the 401/407 challenge loop, codec mismatches, the 32-second BYE, lost early media, one-way audio, session-timer expiry. Each with the trace fingerprint that gives it away in thirty seconds.
Read the post →Every header and SDP field a B2BUA SBC rewrites between the access and core legs, why, and the three pathologies that show up in paired traces: missing Record-Route, Contact rewrite gone wrong, and the silent-PRACK case.
Read the post →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.
Read the post →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.
Read the post →New posts publish on a weekly rotation: SIP · Troubleshooting · SS7/SIGTRAN/RTP · SBCs/Carrier. Subscribe via RSS to keep up.