Notes on SIP

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The Problem of Finding a Peer on the Internet

When two devices need to communicate with each other, they both need to know the other’s IP address.
In some cases, you can subscribe to a static IP from your provider, though this usually involves an additional cost and is not always available (example: cooperatives that provide private subnets). To overcome these limitations, signaling protocols were developed, among which the most important is: SIP (Session Initiation Protocol).

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What is SIP?

SIP is a communication protocol between devices connected to the Internet.
Its primary function is to allow two devices to find each other and establish a session without previously knowing their IP addresses.

• Each SIP client has an address formatted similarly to an email:
  sip:[email protected]
• Both clients must register with a SIP Server, which keeps the list of IPs updated.
• The server acts as an “initial intermediary,” but data transfer is point-to-point between clients.
• The system validates identity via username + password.

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Basic SIP Call Flow

1. Client A sends an INVITE to the SIP Server.
2. The server relays the request to B.
3. B responds with RINGING → the server relays it to A.
4. If B accepts, it sends OK → relayed to A.
5. Both clients receive the IP addresses and session parameters (SDP: sample rate, channels, codec).
6. Audio/data flows directly between A and B.
7. To end, a BYE → OK is sent.

In scenarios with routers/firewalls, NAT intervenes, adding complexity to the connection.

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How do I get a SIP account?

There are many providers (free and paid).
Some offer limited free accounts; others include minutes to conventional phone service through a monthly subscription.

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Practical Guide

You can consult the official guide:
SIP Account Configuration Guide in Linphone When creating an account, the provider supplies the following data:

1. SIP Address
   Example:
   • sip:[email protected]
     Sometimes the fields are provided separately (username + server).
     Also called SIP identity, SIP URL, or SIP URI.
2. Domain
   Example:
   • sip:radiovirtual.linphone.org
   • sip:radiovirtual.onsip.com
     Also known as URL of Registrar or Reg Server.
3. Realm (Authentication realm)
   • May be the same as the domain.
   • Usually configured as “*”.
4. Username / Auth Username
   • Some systems differentiate between a generic username and an authentication username.
   • Example:
     • Username: Luis_Garcia
     • Auth Username: lgarcia77
   • If the provider supplies a single piece of data → both are the same.
5. Password
   • Password provided for validation against the server.

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Example SIP Addresses

sip:[email protected] sip:[email protected]

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Other theoretical definitions

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IP Address

• Every device connected to the Internet has an IP address.
• Format: four numbers separated by dots (e.g., 192.168.0.100).
• The hierarchical organization allows for message routing.

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Public IP vs Private IP

• Within a LAN, devices use private IPs, which are not externally visible.
• The router assigns a public IP to the entire network and acts as a firewall.

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NAT (Network Address Translation)

• Translates private IPs on the LAN to the router’s public IP.
• Allows multiple devices to share a single external IP.
• Different types of NAT exist, which can affect SIP.

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STUN

Session Traversal Utilities for NAT
Allows a client behind NAT to discover its public IP and the type of NAT in use.
Defined in RFC 5389.

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TURN

Traversal Using Relays around NAT
Uses relay servers for clients behind symmetric NAT.
Defined in RFC 8656.

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ICE

Interactive Connectivity Establishment
Combines NAT traversal techniques (STUN, TURN) to establish a connection between two devices on different LANs.
Defined in RFC 8445.