SIP elements and addresses

3.1 User agents: UAC and UAS

A user agentuser agent. An endpoint that sends and receives SIP, such as a phone, a softphone, or a gateway. is an endpoint: a desk phone, a softphone, a mobile app, or a gateway to the telephone network. It starts and ends calls, and it sends and receives the media.

SIP gives a user agent one of two roles, and the role belongs to one transaction — one request and its responses:

• The UACUAC. The user agent role that sends a request. (user agent client) sends a request.
• The UASUAS. The user agent role that receives a request and sends the responses. (user agent server) answers it.

So “UAC” does not mean “the caller”. In one call, Alice’s phone is the UAC for the INVITE. When Bob hangs up, Bob’s phone sends the BYE: now Bob’s phone is the UAC, and Alice’s phone is the UAS. Step through the call and watch the roles swap.

3.2 Proxies

A proxyproxy. An element that forwards requests and responses. It does not start or end calls. sits between user agents. It receives a request, decides where it goes next, and forwards it. It never starts or ends a call, and it never answers on behalf of the callee — except to reject a request or to ask for credentials.

RFC 3261 §16.6 lists the steps a proxy follows for each request it forwards. The main ones:

1. Copies the request.
2. Replaces the Request-URI with the next target — for example, Bob’s AOR becomes the address of Bob’s phone.
3. Decreases Max-Forwards by one.
4. Adds Record-Route, if it wants to stay in the path of later requests (optional).
5. Adds other header fields, if its policy wants (optional).
6. Processes the Route headers.
7. Chooses the next hop: address, port, and transport.
8. Adds its own Via at the top.

Everything else stays as it is. In particular, a proxy must not touch the body:

Stateless, stateful, and dialog-stateful

Proxies differ in what they remember.

Compare the two. In both flows, Proxy B changes the Request-URI and adds a Via. Only the stateful proxy answers 100 Trying itself. In this example, it also adds Record-Route — so the ACK passes through it too.

A proxy that sends one request to several targets — forkingforking. A proxy sends one request to several Contact addresses, in parallel or one after another. — must keep state, because it must collect all the responses and choose the best one:

3.3 Registrar, location service, and redirect server

How does Proxy B know where Bob’s phone is? Bob’s phone told it. When the phone starts, it sends a REGISTER request to the RegistrarRegistrar. The server that accepts REGISTER requests for a domain and stores the bindings in its location service. In most examples it serves atlanta.example; in Module 12, biloxi.example. of its domain.

The registrar stores a bindingbinding. One entry in the location service that maps an AOR to a Contact.: “requests for sip:bob@biloxi.example go to sip:bob@203.0.113.20:5060”. The database of bindings is the location servicelocation service. The database of bindings from AORs to Contact addresses.. Each binding expires, so the phone must register again before the time runs out. Module 12 covers registration in detail.

A Redirect serverRedirect server. A UAS that answers a request with a 3xx response and a list of Contact addresses to try. also uses the location service, but it does not forward the request. It answers with a 3xx response that lists the Contact addresses, and the caller sends a new request itself.

3.4 The B2BUA: two user agents back to back

A B2BUAB2BUA. An element that ends one call leg as a UAS and starts a new leg as a UAC. (back-to-back user agent) is not a proxy. It answers the incoming INVITE as a UAS, and then starts a new INVITE as a UAC. Between the two, its own logic decides what to copy: a PBX dial plan, a prepaid platform, a call recorder.

Compare a proxy and a B2BUA on the same call. Select F3 in each, and look at the changes since F1. The proxy changes four lines. The B2BUA changes almost everything: Call-ID, From tag, CSeq, Via, and Contact are new.

The result is two dialogs, one on each side. Alice’s phone and Bob’s phone never see each other’s Call-ID, tags, or Contact.

Kinds of B2BUA

RFC 7092 names the common kinds, from the most transparent to the least:

3.5 The SBC: a B2BUA at the border

A SBCSBC. Session Border Controller. A B2BUA at the edge of a network that protects and normalises SIP. (Session Border Controller) is a B2BUA at the edge of a network: between an enterprise and its SIP trunk provider, or between two carriers. “SBC” is a product category, not a standard. RFC 5853 describes what SBCs do in practice:

• Protect the network: access control, rate limits, and protection against floods of requests.
• Hide the topology: no inner address leaves the network (topology hidingtopology hiding. An SBC removes or replaces the addresses of the inner network in each message it forwards.).
• Fix what the endpoints cannot: NAT traversal (Module 20), and protocol repair between equipment that does not understand each other.
• Control the media: relay the RTP, measure its quality, and sometimes transcode it.

Watch an SBC forward Alice’s INVITE out of atlanta.example. In F3, select the changes since F1: the Vias and Record-Route of the inner network are gone, and the SDP points to the SBC.

All the elements, side by side

Select an element to see the message it receives, the message it sends, and every line it changes. Select an arrow, or use the buttons, to switch between the two messages.

3.6 SIP URIs

SIP names users, devices, and servers with a URIURI. A text address for a resource, such as sip:alice@atlanta.example or tel:+12025550123.. The general form of a SIP URISIP URI. A URI with the sip scheme. It names a user or a server, such as sip:alice@atlanta.example. is:

Three schemes are common in SIP:

• sip: — the normal scheme. Any transport.
• sips: — a SIPS URISIPS URI. A URI with the sips scheme. It requires TLS on every hop up to the target domain.: the request must use TLS on every hop.
• tel: — a tel URItel URI. A URI that holds a telephone number, such as tel:+12025550123.: a telephone number, with no host.

Type a URI, or pick an example. You can also paste a header value with a display namedisplay name. The optional human-readable name before a URI, such as "Alice" in Alice <sip:alice@atlanta.example>. and angle brackets, such as the value of a To header. Select a part to read about it.

Rules worth remembering

Angle brackets decide who owns a parameter. In <sip:bob@biloxi.example;transport=tcp>;tag=314159, transport is inside the brackets, so it belongs to the URI. tag is outside, so it belongs to the header. Module 4 shows what goes wrong without the brackets.

The user part is case-sensitive; the rest is not. sip:Alice@ATLANTA.example and sip:alice@atlanta.example are different URIs, because the user parts differ.

sips: means TLS on every hop, not only on the first one:

RFC 3261 allowed one exception for the last hop. RFC 5630 removed it:

Phone numbers can appear in two ways: as a tel URI, or as the user part of a SIP URI with ;user=phone. A global number starts with + and the country code. A local number, such as an extension, needs a phone-context:

3.7 Address of Record and Contact

Every user has two kinds of SIP address:

• The AORAOR. The public SIP address of a user, such as sip:alice@atlanta.example. (address of record) is the public address of the user, like an email address: sip:bob@biloxi.example. It appears on business cards, and in the To and From headers. It does not change when Bob moves.
• A ContactContact. The address where a user agent can be reached directly at this time. address is where one device of the user can be reached now: sip:bob@203.0.113.20:5060. It contains an IP address or a host name, and often a port. It changes when the device moves, restarts, or gets a new address.

The registrar joins the two: each REGISTER adds a binding from the AOR to a Contact. One AOR can have several Contacts — a desk phone, a softphone, and a mobile app. Register and unregister Bob’s devices, and see where Proxy B sends Alice’s INVITE.

Common mistakes

Summary

• A user agent is an endpoint. It is the UAC when it sends a request and the UAS when it answers. The role lasts for one transaction.
• A proxy routes requests. It changes the Request-URI and Max-Forwards, adds a Via, and may add Record-Route — nothing else, and never the body. Stateless proxies remember nothing; stateful proxies remember transactions and can fork.
• A registrar stores bindings from AORs to Contacts in the location service. A redirect server answers with 3xx and stays out of the path.
• A B2BUA ends one call and starts another: new Call-ID, tags, CSeq, Via, and Contact. An SBC is a B2BUA at the network border; it usually hides the topology and relays the media.
• A SIP URI has the form sip:user:password@host:port;parameters?headers. The user part is case-sensitive. sips: requires TLS on every hop. tel: holds a phone number.
• The AOR names a user; a Contact names a device. One AOR can have several Contacts, and a proxy forks to all of them.