How SIP Works
SIP is a simple, ASCII-based protocol that uses requests and responses to establish communication
among the various components in the network and to ultimately establish a
conference between two or more endpoints.
Users in a SIP network are identified by unique SIP addresses. A SIP address is similar to
an e-mail address and is in the format of sip:userID@gateway.com. The user ID can be
either a username or an E.164 address. The gateway can be either a domain (with or without
a hostname) or a specific IP address.
Users register with a registrar server using their assigned SIP addresses. The registrar
server provides this information to a location server upon request.
When a user initiates a call, a SIP request is sent to a SIP server (either a proxy or a redirect
server). The request includes the address of the caller (in the From header field) and
the address of the intended called party (in the To header field).
Over time, a SIP end user might move between end systems. The location of the end user
can be dynamically registered with the SIP server. The location server can use one or
more protocols (including finger, rwhois, and LDAP) to locate the end user. Because the
end user can be logged in at more than one station and because the location server can
sometimes have inaccurate information, it might return more than one address for the end
user. If the request is coming through a SIP proxy server, the proxy server tries each of
the returned addresses until it locates the end user. If the request is coming through a SIP
redirect server, the redirect server forwards all the addresses to the caller in the Contact
header field of the invitation response.
Note An E.164 address is a telephone number with a string of decimal digits that
uniquely indicates the public network termination point. The number contains the information
necessary to route the call to this termination point.