VoIP and where SIP fits

1.1 From circuits to packets

For a hundred years, a phone call meant a circuit. The PSTNPSTN. The traditional telephone network, built on circuits between telephone exchanges. — the public telephone network — connects your phone to a telephone exchange, and exchanges connect to each other over trunks. When you dial, the exchanges reserve a path through the network for your call. In the digital telephone network, that path is a channel of 64 kbit/s: 8,000 samples per second, 8 bits each. The channel stays yours until you hang up — even while nobody speaks.

The exchanges agree on the path with their own signaling protocol, ISUPISUP. The SS7 protocol that sets up and ends calls between telephone exchanges., on a separate network called SS7SS7. The signaling network of the PSTN. Telephone exchanges use it to set up calls.. Signaling on one network, voice on another: this idea survives in SIP.

VoIPVoIP. Voice calls carried as packets over an IP network. SIP is one way to set them up. takes another approach. The phone cuts the voice into small pieces — usually 20 milliseconds each — and sends each piece as a packet. Packets from many calls, web pages, and emails share the same links. Nothing is reserved.

Try it: add calls, and make one caller silent.

What VoIP gains:

• Cost and flexibility. One IP network carries voice, video, and data. A phone works wherever it has an IP connection.
• New services. Video, presence, messaging, and web integration use the same protocols as voice.

What VoIP must solve, because nothing is reserved:

• Delay, jitter, and loss. Packets can arrive late, unevenly, or not at all (Modules 16 and 29).
• NAT and firewalls. Home and office routers change addresses and block unknown traffic (Module 20).
• Security. Anyone on the Internet can try to make calls on your account (Module 14).

1.2 Signaling protocols side by side

Remember the two jobs from Module 0: signaling sets up the call, media carries the voice. Almost every VoIP system uses RTP for media. The signaling protocols differ — in who they come from, how they encode messages, and above all in who controls the call.

Why did SIP become the main protocol? It is text, so a person can read a trace. It reuses ideas from HTTP and email, which Internet developers already knew. Its core is small, and it grows through extensions. And when the mobile industry (3GPP) chose a protocol for the IP Multimedia Subsystem — the system behind VoLTE — it chose SIP. Module 27 covers IMS.

1.3 SIP’s design

SIP follows a few clear ideas. Each one explains something that you will see in traces.

1. SIP is text. You can read every SIP message without a decoder. That is why this course shows raw messages everywhere.

2. SIP looks like HTTP. Request line, headers, an empty line, a body; status codes such as 200 OK and 404 Not Found. Point at any line below to find its partner.

3. Requests and responses. Every exchange is a request and its responses: a transaction (Module 8).

4. Addresses like email. A user has a URI such as sip:alice@atlanta.example, independent of the device or the location.

5. Intelligence at the edges. A SIP phone is a full user agentuser agent. An endpoint that sends and receives SIP, such as a phone, a softphone, or a gateway.: it creates requests, answers them, and keeps the state of its calls. The same device acts as client (UAC) for one request and server (UAS) for the next.

6. Primitives, not services. SIP has no “transfer button” or “call waiting” built in. It has building blocks — INVITE, REFER, re-INVITE, NOTIFY — and services are built from them.

7. Any transport. SIP runs over UDP, TCP, TLS, and WebSocket (Module 19).

8. Extensible. New methods, headers, and option tags are added by new RFCs. Two devices find out what the other supports with headers such as Allow and Supported (Module 5).

Where SIP is not like HTTP

The family resemblance is real, but the differences cause many misunderstandings:

1.4 The RFC family

SIP is not one document. RFC 3261 defines the core, and well over a hundred other RFCsRFC. A numbered document from the IETF and other groups. The SIP standards are RFCs. extend it. Some fix it, some add methods or headers, and some replace older documents completely.

Every RFC has a fixed number and a status:

• Proposed Standard — a standard, in practice. Most SIP RFCs, including RFC 3261, have this status, and they run the world’s phone networks.
• Best Current Practice (BCP) — advice on how to do something well. RFC 3665, the call-flow examples, is a BCP. It is still not the specification, and some of its examples have errors (see Module 13).
• Informational — useful information, not a requirement. For example, RFC 6314 describes NAT traversal practices for SIP.
• Historic — no longer in use.

RFCs never change after publication. Instead, a new RFC updates an old one (it changes some parts) or obsoletes it (it replaces it completely). Errors that people find are listed as errata: see the errata for RFC 3261.

Explore the RFCs that this course uses. Select an RFC to see what it does, what replaced it, and which module teaches it. Dashed boxes are RFCs that a newer RFC replaced.

Reading MUST, SHOULD, and MAY

RFCs use a few words with a precise meaning, defined in RFC 2119. They matter: a device that ignores a MUST is broken; a device that ignores a SHOULD may have a good reason.

This course highlights these key words in every RFC quote.

1.5 Where SIP runs today

SIP is in more places than desk phones. You meet it in all of these:

Common mistakes

Summary

• The PSTN reserves a 64 kbit/s circuit for each call. VoIP sends the voice as packets — about 50 per second — on links shared with other traffic.
• SIP is one of several signaling protocols. H.323 and ISUP come from the telephone world; MGCP and H.248 control gateways; IAX2 links Asterisk servers.
• SIP is text, looks like HTTP, keeps the intelligence in the endpoints, and provides primitives rather than services.
• SIP is not HTTP: it usually runs over UDP, both sides send requests, one request can get several responses, and calls keep state.
• SIP is a family of RFCs. Check each RFC’s status, whether it is obsoleted, and its errata. Only capitalised MUST, SHOULD, and MAY are requirements.