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Glossary · Voice technology

What is SIP (session initiation protocol)?

  • Also called: session initiation protocol, sip protocol

Reviewed by Sujan ThapaliyaLast updated

Definition

SIP is the signalling protocol that sets up, changes and ends voice calls over IP networks. It negotiates who is calling whom and which codecs to use; the audio itself travels separately over RTP once the call is established.
The three STIR/SHAKEN attestation levels, which underpin how sip (session initiation protocol) behaves in practice: A asserts both that the carrier knows the customer and that the customer may use the calling number, B asserts the customer only, and C asserts neither. A shared number pool can reach B at best, which is why number ownership keeps recurring in these definitions.
Attestation is the backdrop to most caller-ID and deliverability terms, sip (session initiation protocol) included.

In practice

How sip (session initiation protocol) actually works

Separating signalling from media is why VoIP problems split cleanly in two. If a call fails to connect, the problem is in SIP; if it connects and sounds terrible, the problem is in the media path.

SIP is text-based and readable, which makes call flows diagnosable from a trace — the responses look like web status codes, and a 403 or 404 in a trace means much what it does on the web.

Most residential and small-business routers interfere with SIP through an application-layer gateway that rewrites messages badly. Turning that feature off resolves a surprising share of one-way-audio complaints.

Worked example

A call that fails to connect is a signalling problem; a call that connects and sounds terrible is a media problem. Knowing which halves the diagnosis before you start.

SIP (session initiation protocol): common questions

What does SIP actually do on a call?
It sets the call up and takes it down: locating the far end, agreeing the codecs, ringing, answering, hanging up. It does not carry the audio.
Why do calls connect but have no audio?
Because signalling succeeded and media did not. That is almost always a NAT or firewall problem on the media path, or a router rewriting SIP messages through its ALG.
Is SIP the same as VoIP?
No. VoIP is the general idea of voice over IP; SIP is the specific protocol most systems use to set those calls up. Other signalling protocols exist but SIP dominates.

Sources

  1. Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.
  2. 47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
  3. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

See sip (session initiation protocol) in the product

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