rfc3050.txt
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Network Working Group J. Lennox
Request for Comments: 3050 H. Schulzrinne
Category: Informational Columbia U.
J. Rosenberg
dynamicsoft
January 2001
Common Gateway Interface for SIP
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
In Internet telephony, there must be a means by which new services
are created and deployed rapidly. In the World Wide Web, the Common
Gateway Interface (CGI) has served as popular means towards
programming web services. Due to the similarities between the
Session Initiation Protocol (SIP) and the Hyper Text Transfer
Protocol (HTTP), CGI is a good candidate for service creation in a
SIP environment. This document defines a SIP CGI interface for
providing SIP services on a SIP server.
IESG Note
The IESG notes that the mechanism specified here depends on the
Common Gateway Interface. Should this interface change or be
enhanced changes in this specification may also be necessary or
appropriate. According to the W3C, the CGI is presently maintained
by the NCSA Software Development Group. See
http://www.w3c.org/cgi
for additional information on the current state of the CGI interface.
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RFC 3050 CGI for SIP January 2001
Table of Contents
1 Introduction ....................................... 3
2 Motivations ........................................ 4
3 Differences from HTTP CGI .......................... 5
3.1 Basic Model ........................................ 6
3.2 Persistence Model .................................. 8
3.3 SIP CGI Triggers ................................... 9
3.4 Naming ............................................. 9
3.5 Environment Variables .............................. 9
3.6 Timers ............................................. 10
4 Overview of SIP CGI ................................ 10
5 SIP CGI Specification .............................. 12
5.1 Introduction ....................................... 12
5.1.1 Relationship with HTTP CGI ......................... 12
5.1.2 Conventions of This Document ....................... 12
5.1.3 Specifications ..................................... 12
5.1.4 Terminology ........................................ 13
5.2 Notational Conventions and Generic Grammar ......... 13
5.3 Invoking the Script ................................ 14
5.4 The SIP CGI Script Command Line .................... 14
5.5 Data Input to the SIP CGI Script ................... 14
5.5.1 Message Metadata (Metavariables) ................... 14
5.5.1.1 AUTH_TYPE .......................................... 16
5.5.1.2 CONTENT_LENGTH ..................................... 16
5.5.1.3 CONTENT_TYPE ....................................... 17
5.5.1.4 GATEWAY_INTERFACE .................................. 17
5.5.1.5 Protocol-Specific Metavariables .................... 18
5.5.1.6 REGISTRATIONS ...................................... 18
5.5.1.7 REMOTE_ADDR ........................................ 19
5.5.1.8 REMOTE_HOST ........................................ 19
5.5.1.9 REMOTE_IDENT ....................................... 19
5.5.1.10 REMOTE_USER ........................................ 20
5.5.1.11 REQUEST_METHOD ..................................... 20
5.5.1.12 REQUEST_TOKEN ...................................... 21
5.5.1.13 REQUEST_URI ........................................ 21
5.5.1.14 RESPONSE_STATUS .................................... 21
5.5.1.15 RESPONSE_REASON .................................... 21
5.5.1.16 RESPONSE_TOKEN ..................................... 21
5.5.1.17 SCRIPT_COOKIE ...................................... 22
5.5.1.18 SERVER_NAME ........................................ 22
5.5.1.19 SERVER_PORT ........................................ 22
5.5.1.20 SERVER_PROTOCOL .................................... 22
5.5.1.21 SERVER_SOFTWARE .................................... 23
5.5.2 Message Bodies ..................................... 23
5.6 Data Output from the SIP CGI Script ................ 23
5.6.1 CGI Action Lines ................................... 25
5.6.1.1 Status ............................................. 25
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RFC 3050 CGI for SIP January 2001
5.6.1.2 Proxy Request ...................................... 25
5.6.1.3 Forward Response ................................... 26
5.6.1.4 Script Cookie ...................................... 26
5.6.1.5 CGI Again .......................................... 27
5.6.1.6 Default Action ..................................... 27
5.6.2 CGI Header Fields .................................. 28
5.6.2.1 Request-Token ...................................... 28
5.6.2.2 Remove ............................................. 28
5.7 Local Expiration Handling .......................... 28
5.8 Locally-Generated Responses ........................ 29
5.9 SIP CGI and REGISTER ............................... 29
5.10 SIP CGI and CANCEL ................................. 29
5.11 SIP CGI and ACK .................................... 30
5.11.1 Receiving ACK's .................................... 30
5.11.2 Sending ACK's ...................................... 30
6 System Specifications .............................. 30
6.1 Unix ............................................... 30
6.2 Microsoft Windows .................................. 31
7 Security Considerations ............................ 31
7.1 Request Initiation ................................. 31
7.2 Authenticated and Encrypted Messages ............... 31
7.3 SIP Header Fields Containing Sensitive Information.. 32
7.4 Script Interference with the Server ................ 32
7.5 Data Length and Buffering Considerations ........... 32
8 Acknowledgements ................................... 33
9 Authors' Addresses ................................. 33
10 Bibliography ....................................... 34
11 Full Copyright Statement ........................... 35
1 Introduction
In Internet telephony, there must be a means by which new services
are created and deployed rapidly. In traditional telephony networks,
this was accomplished through IN service creation environments, which
provided an interface for creating new services, often using GUI-
based tools.
The WWW has evolved with its own set of tools for service creation.
Originally, web servers simply translated URLs into filenames stored
on a local system, and returned the file content. Over time, servers
evolved to provide dynamic content, and forms provided a means for
soliciting user input. In essence, what evolved was a means for
service creation in a web environment. There are now many means for
creation of dynamic web content, including server side JavaScript,
servlets, and the common gateway interface (CGI) [1].
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RFC 3050 CGI for SIP January 2001
Multimedia communications, including Internet telephony, will also
require a mechanism for creating services. This mechanism is
strongly tied to the features provided by the signaling protocols.
The Session Initiation Protocol (SIP) [2] has been developed for
initiation and termination of multimedia sessions. SIP borrows
heavily from HTTP, inheriting its client-server interaction and much
of its syntax and semantics. For this reason, the web service
creation environments, and CGI in particular, seem attractive as
starting points for developing SIP based service creation
environments.
2 Motivations
CGI has a number of strengths which make it attractive as an
environment for creating SIP services:
Language independence: CGI works with perl, C, VisualBasic, tcl,
and many other languages, as long as they support access to
environment variables.
Exposes all headers: CGI exposes the content of all the headers
in an HTTP request to the CGI application. An application
can make use of these as it sees fit, and ignore those it
doesn't care about. This allows all aspects of an HTTP
request to be considered for creation of content. In a SIP
environment, headers have greater importance than in HTTP.
They carry critical information about the transaction,
including caller and callee, subject, contact addresses,
organizations, extension names, registration parameters and
expirations, call status, and call routes, to name a few.
It is therefore critical for SIP services to have as much
access to these headers as possible. For this reason, CGI
is very attractive.
Creation of responses: CGI is advantageous in that it can create
all parts of a response, including headers, status codes
and reason phrases, in addition to message bodies. This is
not the case for other mechanisms, such as Java servlets,
which are focused primarily on the body. In a SIP
environment, it is critical to be able to generate all
aspects of a response (and, all aspects of new or proxied
requests), since the body is usually not of central
importance in SIP service creation.
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RFC 3050 CGI for SIP January 2001
Component reuse: Many of the CGI utilities allow for easy
reading of environment variables, parsing of form data, and
often parsing and generation of header fields. Since SIP
reuses the basic RFC822 [3] syntax of HTTP, many of these
tools are applicable to SIP CGI.
Familiar environment: Many web programmers are familiar with
CGI.
Ease of extensibility: Since CGI is an interface and not a
language, it becomes easy to extend and reapply to other
protocols, such as SIP.
The generality, extensibility, and detailed control and access to
information provided by CGI, coupled with the range of tools that
exist for it, which can be immediately applied to SIP, make it a good
mechanism for SIP service creation.
3 Differences from HTTP CGI
While SIP and HTTP share a basic syntax and a request-response model,
there are important differences. Proxies play a critical role in
services for SIP, while they are less important for HTTP. SIP
servers can fork requests (proxying multiple requests when a single
request is received), an important capability absent from HTTP. SIP
supports additional features, such as registrations, which are absent
from HTTP. These differences are reflected in the differences
between SIP CGI and HTTP CGI. SIP CGI runs primarily on proxy,
redirect, and registrar servers, rather than user agent servers
(which are the equivalent of origin servers in HTTP). SIP CGI allows
the script to perform specific messaging functions not supported in
HTTP CGI (such as proxying requests), and SIP CGI introduces a
persistence model that allow a script to maintain control through
multiple message exchanges. HTTP CGI has no persistence for scripts.
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RFC 3050 CGI for SIP January 2001
3.1 Basic Model
The basic model for HTTP CGI is depicted in figure 1.
----- ------------
~~~~~~~~ |req | | -------- |
| |----------| | http | |
| client | |resp | | | server | |
| |----------| | | |w
~~~~~~~~ | | | -------- |e
----- | s| /\s |b
net | t| |t |
|e d| C |d |s
|n i| G |o |e
|v n| I |u |r
| | |t |v
| \/ | |e
| ------- |r
| | | |
| | CGI | |
| | prog. | |
| | | |
| ------- |
------------
Figure 1: HTTP CGI Model
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