Table 7.1 (continued)
SERVICE
IMPLEMENTATION ADVANTAGES DISADVANTAGES
Calling user agent No logic or provisioning in Requires feature logic in calling
called user agent, which user agent. Only works if SIP
does not need to be server redirects instead of
registered. Caller has the proxying. Since this is not under
choice of connecting to the control of the caller, this
voicemail. service will not always work in
a reliable way.New Methods and Headers
New features and services can be implemented in SIP by defining new meth-
ods or headers. The basic set of methods and headers are defined in the SIP
base specification, which covers basic session establishment and some features
and services. New headers and methods can be proposed in the IETF through
a process of writing and submitting an Internet draft document. If this docu-
ment fits the chartered scope of the working group and gathers sufficient sup-
port, it may be adopted by the working group as an official work item. The
status of the Internet draft is then tracked on the working group charter page
as it is discussed and reviewed. Eventually, the document may become an RFC
and an official extension to SIP. Nearly all the SIP extensions referenced in this
book are RFCs or are official work items of the SIP, SIPPING, or SIMPLE work-
ing groups, and are likely to become RFCs in the near future.
It is important to note that new methods or headers do not need support by
SIP servers. For example, a SIP proxy receiving a request with an unknown
method will proxy the request, treating it as if it were an OPTIONSrequest. A
SIP proxy that receives a request with an unknown header will simply proxy
the request, making no change to the header. Only the presence of a Proxy-
Requireheader will force a proxy to understand and take action based on a
particular header or method.
This allows new services to be created in user agents and deployed without
any changes in the SIP network. Note that this is essentially what has hap-
pened with SIP-enabled telephony networks that can provide SIP instant mes-
sage and presence transport without any changes to the SIP infrastructure.
Many new methods can be defined without having to use the Supported
header. For example, a user agent sending an INFOrequest to a user agent that
does not support this extension to SIP will receive a 405 Method Not
Allowed(if it recognizes the method but does not support it) or a 500 Bad
Request(if it does not recognize the method) response with an Allowheader
SIP Service Creation 141