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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Table of Contents
1 Introduction ....................................... 3
2 Motivations ........................................ 4
3 Differences from HTTP CGI .......................... 53.1 Basic Model ........................................ 63.2 Persistence Model .................................. 83.3 SIP CGI Triggers ................................... 93.4 Naming ............................................. 93.5 Environment Variables .............................. 93.6 Timers ............................................. 10
4 Overview of SIP CGI ................................ 10
5 SIP CGI Specification .............................. 125.1 Introduction ....................................... 125.1.1 Relationship with HTTP CGI ......................... 125.1.2 Conventions of This Document ....................... 125.1.3 Specifications ..................................... 125.1.4 Terminology ........................................ 135.2 Notational Conventions and Generic Grammar ......... 135.3 Invoking the Script ................................ 145.4 The SIP CGI Script Command Line .................... 145.5 Data Input to the SIP CGI Script ................... 145.5.1 Message Metadata (Metavariables) ................... 145.5.1.1 AUTH_TYPE .......................................... 165.5.1.2 CONTENT_LENGTH ..................................... 165.5.1.3 CONTENT_TYPE ....................................... 175.5.1.4 GATEWAY_INTERFACE .................................. 175.5.1.5 Protocol-Specific Metavariables .................... 185.5.1.6 REGISTRATIONS ...................................... 185.5.1.7 REMOTE_ADDR ........................................ 195.5.1.8 REMOTE_HOST ........................................ 195.5.1.9 REMOTE_IDENT ....................................... 195.5.1.10 REMOTE_USER ........................................ 205.5.1.11 REQUEST_METHOD ..................................... 205.5.1.12 REQUEST_TOKEN ...................................... 215.5.1.13 REQUEST_URI ........................................ 215.5.1.14 RESPONSE_STATUS .................................... 215.5.1.15 RESPONSE_REASON .................................... 215.5.1.16 RESPONSE_TOKEN ..................................... 215.5.1.17 SCRIPT_COOKIE ...................................... 225.5.1.18 SERVER_NAME ........................................ 225.5.1.19 SERVER_PORT ........................................ 225.5.1.20 SERVER_PROTOCOL .................................... 225.5.1.21 SERVER_SOFTWARE .................................... 235.5.2 Message Bodies ..................................... 235.6 Data Output from the SIP CGI Script ................ 235.6.1 CGI Action Lines ................................... 255.6.1.1 Status ............................................. 25
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5.6.1.2 Proxy Request ...................................... 255.6.1.3 Forward Response ................................... 265.6.1.4 Script Cookie ...................................... 265.6.1.5 CGI Again .......................................... 275.6.1.6 Default Action ..................................... 275.6.2 CGI Header Fields .................................. 285.6.2.1 Request-Token ...................................... 285.6.2.2 Remove ............................................. 285.7 Local Expiration Handling .......................... 285.8 Locally-Generated Responses ........................ 295.9 SIP CGI and REGISTER ............................... 295.10 SIP CGI and CANCEL ................................. 295.11 SIP CGI and ACK .................................... 305.11.1 Receiving ACK's .................................... 305.11.2 Sending ACK's ...................................... 30
6 System Specifications .............................. 306.1 Unix ............................................... 306.2 Microsoft Windows .................................. 31
7 Security Considerations ............................ 317.1 Request Initiation ................................. 317.2 Authenticated and Encrypted Messages ............... 317.3 SIP Header Fields Containing Sensitive Information.. 32
7.4 Script Interference with the Server ................ 327.5 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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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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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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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
A client issues an HTTP request, which is passed either directly to
the origin server (as shown), or is forwarded through a proxy server.
The origin server executes a CGI script, and the CGI script returns a
response, which is passed back to the client. The main job of the
script is to generate the body for the response. Only origin servers
execute CGI scripts, not proxy servers.
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In a SIP server, the model is different, and is depicted in Figure 2.
~~~~~~~~ req ------- req ------- req ~~~~~~~~
| |------| |-------| |---------| |
| client | resp | server| resp | server| resp | client |
| |------| |-------| |---------| |
~~~~~~~~ ------- ------- --------
| | CGI
| |
-------
| |
| CGI |
| prog. |
| |
-------
Figure 2: SIP CGI Model
The client generates a request, which is forwarded to a server. The
server may generate a response (such as an error or redirect
response). Or, if the server is a proxy server, the request is
proxied to another server, and eventually to a user agent, and the
response is passed back upstream, through the server, and back
towards the client. A SIP proxy server may additionally fork
requests, generating multiple requests in response to a received
request. Generally, a proxy server will not generate the content in
responses. These contain session descriptions created by user
agents. Services, such as call forward and mobility services, are
based on the decisions the server makes about (1) when, to where, and
how many requests to proxy downstream, and (2) when to send a
response back upstream. Creation of services such as ad-hoc bridging
(where the server acts as a media mixer in a multiparty call, without
being asked to do so by the end users) will require the server to
generate new requests of its own, and for it to modify and generate
the body in responses.
An HTTP server is mainly concerned about generation of responses. A
SIP server is generally concerned about performing four basic
operations:
Proxying of Requests: Receiving a request, adding or modifying
any of the headers, deciding on a set of servers to forward
the request to, and forwarding it to them.
Returning Responses: Receiving a response, adding or modifying
any of the headers, and passing the response towards the
client.
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Generating Requests: Creating a new request, originating at the
server, placing headers and a body into the message, and
sending it to a server.
Generation of Responses: Receiving a request, generating a
response to it, and sending it back to the client.
When a request is received, one or more of the above operations may
occur at once. For example, a SIP server may generate a provisional
response, generate a new request, and proxy the original request to
two servers. This implies that SIP CGI must encompass a greater set
of functions than HTTP CGI. These functions are a super-set of the
simple end-server request/response model.
In HTTP CGI, a script is executed once for each request. It
generates the response, and then terminates. There is no state
maintained across requests from the same user, as a general rule
(although this can be done -- and is -- for more complex services
such as database accesses, which essentially encapsulate state in
client-side cookies or dynamically-generated URLs). A transaction is
just a single request, and a response.
In SIP CGI, since a request can generate many new and proxied
requests, these themselves will generate responses. A service will
often require these responses to be processed, and additional
requests or responses to be generated. As a result, whereas an HTTP
CGI script executes once per transaction, a SIP CGI script must
maintain control somehow over numerous events.
In order to enable this, and to stay with the original CGI model, we
mandate that a SIP CGI script executes when a message arrives, and
after generating output (in the form of additional messages),
terminate. State is maintained by allowing the CGI to return an
opaque token to the server. When the CGI script is called again for
the same transaction, this token is passed back to the CGI script.
When called for a new transaction, no token is passed.
For example, consider a request which arrives at a SIP server. The
server calls a CGI script, which generates a provisional response and
a proxied request. It also returns a token to the server, and then
terminates. The response is returned upstream towards the client,
and the request is proxied. When the response to the proxied request
arrives, the script is executed again. The environment variables are
set based on the content of the new response. The script is also
passed back the token. Using the token as its state, the script
decides to proxy the request to a different location. It therefore
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returns a proxied request, and another token. The server forwards
this new request, and when the response comes, calls the CGI script
once more, and passes back the token. This time, the script
generates a final response, and passes this back to the server. The
server sends the response to the client, destroys the token, and the
transaction is complete.
In many cases, calling the CGI script on the reception of every
message is inefficient. CGI scripts come at the cost of significant
overhead since they generally require creation of a new process.
Therefore, it is important in SIP CGI for a script to indicate, after
it is called the first time, under what conditions it will be called
for the remainder of the transaction. If the script is not called,
the server will take the "default" action, as specified in this
document. This allows an application designer to trade off
flexibility for computational resources. Making an analogy to the
Intelligent Network (IN) - a script is able to define the triggers
for its future execution.
So, in summary, whereas an HTTP CGI script executes once during a
transaction, a single SIP CGI script may execute many times during a
transaction, and may specify at which points it would like to have
control for the remainder of the transaction.
In HTTP CGI, the CGI script itself is generally the resource named in
the request URI of the HTTP request. This is not so in SIP. In
general, the request URI names a user to be called. The mapping to a
script to be executed may depend on other SIP headers, including To
and From fields, the SIP method, status codes, and reason phrases.
As such, the mapping of a message to a CGI script is purely a matter
of local policy administration at a server. A server may have a
single script which always executes, or it may have multiple scripts,
and the target is selected by some parts of the header.
In HTTP CGI, environment variables are set with the values of the
paths and other aspects of the request. As there is no notion of a
path in SIP, some of these environment variables do not make sense.
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In SIP, certain services require that the script gets called not only
when a message arrives, but when some timer expires. The classic
example of this is "call forward no answer." To be implemented with
SIP CGI, the first time the script is executed, it must generate a
proxied request, and also indicate a time at which to be called again
if no response comes. This kind of feature is not present in HTTP
CGI, and some rudimentary support for it is needed in SIP CGI.
4 Overview of SIP CGI
When a request arrives at a SIP server, initiating a new transaction,
the server will set a number of environment variables, and call a CGI
script. The script is passed the body of the request through stdin.
The script returns, on stdout, a set of SIP action lines, each of
which may be modified by CGI and/or SIP headers. This set is
delimited through the use of two carriage returns. The action lines
allow the script to specify any of the four operations defined above,
in addition to the default operation. Generating a response is done
by copying the the status line of the response into an action line of
the CGI output. For example, the following will create a 200 OK to
the original request:
SIP/2.0 200 OK
The operation of proxying a request is supported by the CGI-PROXY-
REQUEST CGI action, which takes the URL to proxy to as an argument.
For example, to proxy a request to dante@inferno.com:
CGI-PROXY-REQUEST sip:dante@inferno.com SIP/2.0
Contact: sip:server1@company.com
In this example, the server will take the original request, and
modify any header fields normally changed during the proxy operation
(such as decrementing Max-Forwards, and adding a Via field). This
message is then "merged" with the output of the CGI script - SIP
headers specified below the action line in the CGI output will be
added to the outbound request. In the above example, the Contact
header will be added. Note that the action line looks like the
request line of a SIP request message. This is done in order to
simplify parsing.
To delete headers from the outgoing request, the merge process also
supports the CGI header CGI-Remove. Like SIP headers, CGI headers
are written underneath the action line. They are extracted by the
SIP server, and used to provide the server with additional guidance.
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CGI headers always begin with CGI to differentiate them from SIP
headers. In this case, the supported values for the CGI-Remove
header are the names of headers in the original message.
Returning of responses is more complex. A server may receive
multiple responses as the result of forking a request. The script
should be able to ask the server to return any of the responses it
had received previously. To support this, the server will pass an
opaque token to the script through environment variables, unique for
each response received. To return a response, a CGI script needs to
indicate which response is to be returned. For example, to return a
response named with the token abcdefghij, the following output is
generated:
CGI-FORWARD-RESPONSE abcdefghij SIP/2.0
Finally, if the script does not output any of the above actions, the
server does what it would normally do upon receiving the message that
triggered the script.
A SIP CGI script is normally only executed when the original request
arrives. If the script also wants to be called for subsequent
messages in a transaction -- due to responses to proxied requests, or
(in certain circumstances) ACK and CANCEL requests, it can perform
the CGI-AGAIN action:
CGI-AGAIN yes SIP/2.0
This action applies only to the next invocation of the script; it
means to invoke the script one more time. Outputting "no" is
identical to outputting "yes" on this invocation of the script and
outputting nothing the next time the script is called.
When the script is re-executed, it may need access to some state in
order to continue processing. A script can generate one piece of
state, called a cookie, for any new request or proxied request. It
is passed to the server through the CGI-SET-COOKIE action. The
action contains a token, which is the cookie itself. The server does
not examine or parse the cookie. It is simply stored. When the
script is re-executed, the cookie is passed back to the script
through an environment variable.
CGI-SET-COOKIE khsihppii8asdl SIP/2.0
Finally, when the script causes the server to proxy a request,
responses to these requests will arrive. To ease matching of
responses to requests, the script can place a request token in the
CGI CGI-Request-Token header. This header is removed by the server
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when the request is proxied. Any responses received to this request
will have the token passed in an environment variable.
5 SIP CGI Specification
This SIP CGI specification is based on work-in-progress revision 1.1
of the HTTP CGI specification [1]. That document is a product of the
CGI-WG mailing list, which is not an official IETF working group.
CGI-WG's homepage is located at the URL
http://Web.Golux.Com/coar/cgi/, and the most recent versions of the
CGI specification are available there. This specification
incorporates a great deal of text from the work-in-progress version
of that document as of February 23, 2000. A future version of this
specification may be changed to cite parts of that document by
reference instead.
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [4] and
indicate requirement levels for compliant SIP CGI implementations.
Some paragraphs are indented, like this; they give
motivations of design choices, or questions for future
discussion in the development of SIP CGI. They are not
normative to the specification of the protocol.
Not all of the functions and features of SIP CGI are defined in the
main part of this specification. The following phrases are used to
describe the features which are not specified:
System-defined: The feature may differ between systems, but
must be the same for different implementations using the
same system. A system will usually identify a class of
operating systems. Some systems are defined in section 6
of this document. New systems may be defined by new
specifications without revision of this document.
Implementation-defined: The behavior of the feature may vary
from implementation to implementation, but a particular
implementation should be consistent in its behavior.
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This specification uses many terms defined in the SIP/2.0
specification [2]; however, the following terms are used here in a
sense which may not accord with their definitions in that document,
or with their common meaning.
metavariable: A named parameter that carries information from
the server to the script. It is not necessarily a
variable in the operating system's environment, although
that is the most common implementation.
script: The software which is invoked by the server via this
interface. It need not be a standalone program, but
could be a dynamically-loaded or shared library, or even
a subroutine in the server. It may be a set of
statements interpreted at run-time, as the term `script'
is frequently understood, but that is not a requirement
and within the context of this specification the term has
the broader definition stated.
server: The application program which invokes the script in
order to service messages.
message: A SIP request or response, typically either the one
that triggered the invocation of the CGI script, or one
that the CGI script caused to be sent.
In this specification we use the Augmented Backus-Naur Form notation
as described in appendix C of the SIP/2.0 specification, RFC 2543
[2].
The following grammatical constructs are taken from other documents;
this table lists the appropriate sources.
OCTET SIP/2.0 [2] Appendix C.1
CHAR SIP/2.0 [2] Appendix C.1
digit SIP/2.0 [2] Appendix C.1
alphanum SIP/2.0 [2] Appendix C.1
token SIP/2.0 [2] Appendix C.1
hostname SIP/2.0 [2] Section 2
SIP-URL SIP/2.0 [2] Section 2
SIP-Version SIP/2.0 [2] Section 4.3.1
Status-Code SIP/2.0 [2] Section 5.1.1
Reason-Phrase SIP/2.0 [2] Section 5.1.1
media-type HTTP/1.1 [5] Section 3.7
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(via SIP/2.0 [2] Section 6.16)
field-name SIP/2.0 [2] Section 6.6
Other grammatical constructs taken from outside sources are noted in
the text.
The script is invoked in a system-defined manner. Unless specified
otherwise, the file containing the script will be invoked as an
executable program.
Only one CGI script at a time may be outstanding for a SIP
transaction. If subsequently arriving responses would cause a CGI
script to be invoked, handling of them is deferred, except for ACK,
until CGI scripts for previous messages in the transaction terminate.
Messages are processed in the order they are received.
The server SHOULD NOT provide any command line arguments to the
script.
Command line arguments are used for indexed queries in HTTP
CGI; HTTP indexed queries do not have an equivalent in SIP.
Information about a message comes from two different sources: the
message header, and any associated content-body. Servers MUST make
portions of this information available to scripts.
Each SIP CGI server implementation MUST define a mechanism to pass
data about the message from the server to the script. The
metavariables containing these data are accessed by the script in a
system-defined manner. The representation of the characters in the
metavariables is system-defined.
The representation of metavariables MUST distinguish between
undefined values (which are not present) and null values (which are
present, but have zero length). Null values are only allowed for
those metavariables whose grammar permits this.
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For historical reasons, HTTP CGI does not distinguish
between null values and undefined values. This
specification eliminates this misfeature; null values and
undefined values are semantically different.
Case is not significant in the metavariable names, in that there
cannot be two different variables whose names differ in case only.
Here they are shown using a canonical representation of capitals plus
underscore ("_"). The actual representation of the names is system
defined; for a particular system the representation MAY be defined
differently than this.
Metavariable values MUST be considered case-sensitive except as noted
otherwise.
The canonical metavariables defined by this specification are:
AUTH_TYPE
CONTENT_LENGTH
CONTENT_TYPE
GATEWAY_INTERFACE
REMOTE_ADDR
REMOTE_HOST
REMOTE_IDENT
REMOTE_USER
REGISTRATIONS
REQUEST_METHOD
REQUEST_TOKEN
REQUEST_URI
RESPONSE_STATUS
RESPONSE_REASON
RESPONSE_TOKEN
SCRIPT_COOKIE
SERVER_NAME
SERVER_PORT
SERVER_PROTOCOL
SERVER_SOFTWARE
Metavariables with names beginning with the protocol name (e.g.,
"SIP_ACCEPT") are also canonical in their description of message
header fields. The number and meaning of these fields may change
independently of this specification. (See also section 5.5.1.5.)
A server MAY also specify additional non-canonical metavariables.
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If the target of the message required access authentication for
external access, then the server MUST set the value of this variable
from the auth-scheme token in the message's Authorization header
field. Otherwise it is not defined.
AUTH_TYPE = "" | auth-scheme
auth-scheme = "Basic" | "Digest" | "PGP" | token
SIP access authentication schemes are described in sections 14 and 15
of the SIP/2.0 specification [2]. The auth-scheme is not case-
sensitive.
Servers MUST provide this metavariable to scripts if the message
header included an Authorization field that was authenticated.
For the complex authentication schemes, the server SHOULD perform the
authentication checking itself. If the authentication failed, this
metavariable SHOULD NOT be set.
If several authentication credentials, with multiple schemes, are
present in the message, this variable SHOULD be set to correspond to
the authenticated credentials with the strongest scheme the server
supports. If credentials are present for several domains, the server
SHOULD NOT perform any action on credentials from domains external to
it.
If both Authorization and Proxy-Authorization headers are present,
the server SHOULD perform the authorizations based on the appropriate
header for the context in which it is running. For example, a server
which is a proxy server and a registrar would use Authorization
headers for REGISTER messages aimed at its local domains, and Proxy-
Authorization headers for all other messages.
This metavariable is set to the size of the message-body entity
attached to the message, if any, in decimal number of octets. If no
data are attached, then this metavariable is not defined. The syntax
is the same as for the SIP Content-Length header field (section 6.15,
SIP/2.0 specification [2]).
CONTENT_LENGTH = "" | 1*digit
Servers MUST provide this metavariable to scripts if the message was
a accompanied by a content-body entity, even if the message did not
include a Content-Length header field.
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If the message includes a message-body, CONTENT_TYPE is set to the
Internet Media Type [6] of the attached entity if the type was
provided via a Content-type field in the message header, or if the
server can determine it in the absence of a supplied Content-type
field. The syntax is the same as for the SIP Content-Type header
field.
CONTENT_TYPE = "" | media-type
The type, subtype, and parameter attribute names are not case-
sensitive. Parameter values MAY be case sensitive. Media types and
their use in SIP are described in section 6.16 of the SIP/2.0
specification [2], and by reference in section 3.7 of the HTTP/1.1
specification [5].
Since in SIP the Content-Type header MUST be specified if a body is
present, servers MUST provide this metavariable to scripts if a body
was present in the original message, unless the "body" is actually an
encrypted payload.
This metavariable is set to the dialect of SIP CGI being used by the
server to communicate with the script. Syntax:
GATEWAY_INTERFACE = "SIP-CGI" "/" major "." minor
major = 1*digit
minor = 1*digit
Note that the major and minor numbers are treated as separate
integers and hence each may be more than a single digit. Thus SIP-
CGI/2.4 is a lower version than SIP-CGI/2.13 which in turn is lower
than SIP-CGI/12.3. Leading zeros in either the major or the minor
number MUST be ignored by scripts and SHOULD NOT be generated by
servers.
This document defines the 1.1 version of the SIP CGI interface
("SIP-CGI/1.1").
Servers MUST provide this metavariable to scripts.
For maximal compatibility with existing HTTP CGI libraries,
we want to keep this as similar as possible to the syntax
of CGI 1.1. However, we do want it to be clear that this is
indeed SIP CGI. Making HTTP CGI's version identifier a
substring of the SIP CGI identifier seemed like a
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reasonable compromise. (The existing CGI libraries we
checked do not seem to check the version.)
These metavariables are specific to the protocol via which the method
is sent. Interpretation of these variables depends on the value of
the SERVER_PROTOCOL metavariable (see section 5.5.1.20).
Metavariables with names beginning with "SIP_" contain values from
the message header, if the protocol used was SIP. Each SIP header
field name is converted to upper case, has all occurrences of "-"
replaced with "_", and has "SIP_" prepended to form the metavariable
name. Similar transformations are applied for other protocols. The
header data MAY be presented as sent by the client, or MAY be
rewritten in ways which do not change its semantics. If multiple
header fields with the same field-name are received then the server
MUST rewrite them as though they had been received as a single header
field having the same semantics before being represented in a
metavariable. Similarly, a header field that is received on more
than one line MUST be merged into a single line. The server MUST, if
necessary, change the representation of the data (for example, the
character set) to be appropriate for a CGI metavariable.
Note: these metavariables' names were changed from HTTP_*
to SIP_* since the first draft of this specification. The
intention had been to make it easier to use existing CGI
libraries unmodified, but this convenience was felt to be
outweighed by the confusion this introduced.
Servers are not required to create metavariables for all the message
header fields they receive. However, because of the relatively high
importance of headers in SIP for messages' semantic content, the
server SHOULD provide all headers which do not contain potentially
sensitive authorization information, such as Authorization. Servers
SHOULD provide protocol-specific metavariables even for information
which is available through other SIP CGI metavariables, such as
CONTENT_LENGTH and CONTENT_TYPE.
This allows a SIP CGI script to determine, if necessary,
whether the information in the other metavariables was in
the original message, or was synthesized by the server.
This metavariable contains a list the current locations the server
has registered for the user in the Request-URI of the initial request
of a transaction. It is syntactically identical to the protocol
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metavariable SIP_CONTACT, and thus is defined by section 5.5.1.5 of
this document and by section 6.13 of the SIP/2.0 specification [2].
It contains all the uris, uri parameters, display names, and contact
parameters for the addresses registered with the server.
The syntax of REGISTRATIONS is identical to how SIP_CONTACT
would appear in a 302 response from a redirection server.
This allows parsing code to be re-used.
If a user's registrations change in the course of a transaction, the
server SHOULD update this metavariable accordingly for subsequent
script invocations for the transaction.
The IP address of the client that sent the message to the server.
This is not necessarily that of the originating user agent client or
server.
REMOTE_ADDR = hostnumber
hostnumber = IPv4address | IPv6address
The definitions of IPv4address and Ipv6address are provided in
Appendix B of RFC 2373 [7].
For locally-generated responses (see section 5.8), this SHOULD be the
loopback address (i.e., 127.0.0.1 for IPv4 or ::1 for IPv6).
Servers MUST supply this value to scripts.
This is the fully qualified domain name of the host sending the
message to this server, if available, otherwise not defined. (See
section 5.5.1.7). Domain names are not case sensitive.
REMOTE_HOST = hostname
Servers SHOULD provide this information to scripts.
The identity information supported about the connection by a RFC 1413
[8] request, if available.
REMOTE_IDENT = *CHAR
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The server MAY choose not to support this feature, and it is
anticipated that not many implementations will, as the information is
not particularly useful in the presence of complex proxy paths.
If the message requested authentication (i.e., the AUTH_TYPE
metavariable is set), then the value of the REMOTE_USER metavariable
is set to the user-ID supplied for the authentication. For Basic
authentication this is the content of the (decoded) "userid" grammar
element; for Digest it is content of "username-value." For PGP
authentication, it is the URI specified in the "signed-by" parameter
of the Authorization header, if present, otherwise the URI part of
the From header.
If some other authentication scheme was requested, this metavariable
SHOULD be set to an appropriate component of the authorization
information identifying the user or entity associated with the
credentials. If authentication was not requested, this metavariable
is not defined.
REMOTE_USER = *OCTET
Servers SHOULD provide this metavariable to scripts.
If the message triggering the script was a request, the
REQUEST_METHOD metavariable is set to the method with which the
request was made, as described in section 4.2 of the SIP/2.0
specification [2]; otherwise not defined.
REQUEST_METHOD = sip-method
sip-method = "INVITE" | "BYE" | "OPTIONS" | "CANCEL"
| "REGISTER" | "ACK"
| extension-method
extension-method = token
Note that ACK is usually not appropriate for the SIP CGI 1.1
environment; however, see section 5.11. The implications of REGISTER
in the CGI context are discussed in section 5.9, and CANCEL is
discussed in section 5.10. A SIP CGI 1.1 server MAY choose to
process some methods directly rather than passing them to scripts.
Servers MUST provide this metavariable to scripts if the triggering
message was a request.
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REQUEST_TOKEN = token
If the script specified a request token in a proxied request, this
token is returned to the server in responses to that request. Note
that this token is chosen by the script, not by the server. Each
response to a proxied request contains the same value for this token.
This metavariable is specific to requests made with SIP.
REQUEST_URI = absoluteURI ; defined in RFC 2396 [9]
If the message triggering the script was a request, this variable
indicates the URI specified with the request method. This
metavariable is only defined if REQUEST_METHOD is defined; in that
case, servers MUST provide it to scripts.
This metavariable fills the roles of HTTP CGI's
SCRIPT_NAME, PATH_INFO, and QUERY_STRING.
RESPONSE_STATUS = Status-Code
If the message triggering the script was a response, this variable
indicates the numeric code specified in the response; otherwise it is
not defined. In the former case, servers MUST provide this
metavariable to scripts.
RESPONSE_REASON = Reason-Phrase
If the message triggering the script was a response, this variable
indicates the textual string specified in the response.
RESPONSE_TOKEN = token
If the message triggering the script was a response, the server MUST
specify a token which subsequent invocations of the CGI script can
use to identify this response. This string is chosen by the server
and is opaque to the CGI script. See the discussion of CGI-FORWARD-
RESPONSE in section 5.6.1 below.
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SCRIPT_COOKIE = token
This is the value an earlier invocation of this script for this
transaction passed to the server in CGI action line CGI-SET-COOKIE.
See the description of that action in section 5.6.1.4 below.
The SERVER_PORT metavariable is set to the port on which the message
was received.
SERVER_PORT = 1*digit
Servers MUST provide this metavariable to scripts.
The SERVER_PROTOCOL metavariable is set to the name and revision of
the protocol with which the message arrived. This will usually be
"SIP/2.0". This is not necessarily the same as the protocol version
used by the server in its response to the client.
SERVER_PROTOCOL = SIP-Version | extension-version
| extension-token
extension-version = protocol "/" 1*digit "." 1*digit
protocol = 1*( alphanum | "+" | "-" | "." )
extension-token = token
Servers MUST provide this metavariable to scripts.
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The SERVER_SOFTWARE metavariable is set to the name and version of
the information server software handling the message (and running the
gateway).
SERVER_SOFTWARE = 1*product
product = token [ "/" product-version ]
product-version = token
Servers MUST provide this metavariable to scripts.
As there may be a data entity attached to the message, there MUST be
a system-defined method for the script to read these data. Unless
defined otherwise, this will be via the `standard input' file
descriptor.
If the metavariable CONTENT_LENGTH (see section 5.5.1.2) is defined,
the server MUST supply at least that many bytes to scripts on the
standard input stream. Scripts are not obliged to read the data.
Servers MAY signal an EOF condition after CONTENT_LENGTH bytes have
been read, but are not obligated to do so. Therefore, scripts MUST
NOT attempt to read more than CONTENT_LENGTH bytes, even if more data
are available.
There MUST be a system-defined method for the script to send data
back to the server or client. Unless defined otherwise, this will be
via the `standard output' file descriptor.
Servers MAY implement a timeout period within which data must be
received from scripts, a maximum number of requests or responses that
a particular CGI script can initiate, a maximum total number of
requests or responses that can be sent by scripts over the lifetime
of a transaction, or any other resource limitations it desires. If a
script exceeds one of these limitations, the server MAY terminate the
script process and SHOULD abort the transaction with either a `504
Gateway Timed Out' or a `500 Internal Server Error' response.
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A SIP CGI script's output consists of any number of messages, each
corresponding to actions which the script is requesting that the
server perform. Messages consist of an action line, whose syntax is
specific to the type of action, followed by CGI header fields and SIP
header fields. Action lines determine the nature of the action
performed, and are described in section 5.6.1. CGI header fields
pass additional instructions or information to the server, and are
described in section 5.6.2.
A message MUST contain exactly one action line, MAY also contain any
number of CGI header fields and SIP header fields, and MAY contain a
SIP body.
All header fields (both SIP and CGI) occurring in an output message
MUST be specified one per line; SIP CGI 1.1 makes no provision for
continuation lines.
The generic syntax of CGI header fields is specified in section
5.6.2.
A server MAY choose to honor only some of the requests or responses;
in particular, it SHOULD NOT accept any responses following a Status
message which sends a definitive non-success response.
The messages sent by a script are delimited as follows:
1. A message begins with an action line.
2. If the message does not contain a Content-Type header
field, or if it contains the header field "Content-Length:
0", then it is terminated by a blank line.
3. If the message contains both Content-Type and Content-
Length header fields, the message has a body consisting of
the Content-Length octets following the blank line below
the set. The next message begins after the body (and
optionally some number of blank lines). If the script
closes its output prematurely, the server SHOULD report a
500-class server error.
4. If the message contains Content-Type but not Content-
Length, the message's body similarly begins with the blank
line following the set; this body extends until the script
closes its output. In this case, this is necessarily the
last message the script can send. The server SHOULD insert
a Content-Length header containing the amount of data read
before the script closed its output.
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5. If a message contains a non-zero Content-Length but does
not contain a Content-Type, it is an error. The server
SHOULD report a 500-class server error.
The output of a SIP CGI script is intended to be
syntactically identical to that of a UDP packet in which
multiple requests or responses are sent, so that the same
message parser may be used.
Status = SIP-Version 3*digit SP reason-phrase NL
This action line causes the server to generate a SIP response and
relay it upstream towards the client. The server MUST copy the To,
From, Call-ID, and CSeq headers from the original request into the
response if these headers are not specified in the script output.
The server SHOULD copy any other headers from the request which would
normally be copied in the response if these are not specified in the
script output.
For compatibility with HTTP CGI, a server MAY interpret a message
containing a Content-Type header field and no action line as though
it contained "SIP/2.0 200 OK". This usage is deprecated.
Proxy-Request = "CGI-PROXY-REQUEST" SIP-URL SIP-Version
This action line causes the server to forward a request to the
specified SIP URI. It may be sent either by a script triggered by a
request, in which case the triggering request is forwarded; or by a
script triggered by a response on a server which is running
statefully, in which case the initial request of the transaction is
sent.
Any SIP header field MAY be specified below the action line.
Specified SIP headers replace all those in the original message in
their entirety; if a script wants to preserve header elements from
the original message as well as adding new ones, it can concatenate
them by the usual rules of header concatenation, and place the result
in the script output. New header fields are added to the message
after any Via headers but before any other headers.
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Any headers from the original request which are not generated by the
CGI script are copied into the proxied request, after modifications
normally performed by a proxy server. In particular, the server MUST
append a Via field and decrement Max-Forwards. A server MAY perform
additional modifications as it sees fit, such as adding a Record-
Route header. A server SHOULD NOT append these headers if they are
specified in the script output.
A script MAY specify that a SIP header is to be deleted from the
message by using the CGI-Remove CGI header; see section 5.6.2.
If the message does not specify a body, the body from the initial
request is used. A message with "Content-Length: 0" is specifying an
empty body; this causes the body to be deleted from the message.
If the original request was authenticated by any means other than
`basic,' the script SHOULD NOT add, change, or remove any end-to-end
headers, as this would break the authentication.
Forward-Response = "CGI-FORWARD-RESPONSE" Response-Name
SIP-Version
Response-Name = response-token | "this"
This action line causes the server to forward a response on to its
appropriate final destination. The same rules apply for accompanying
SIP headers and message bodies as for CGI-PROXY-REQUEST.
The specified response name may either be a response token the server
previously submitted in a RESPONSE_TOKEN metavariable, or the string
"this." The string "this" may only be sent if the message which
triggered this CGI script was a response; it indicates that this
triggering response should be forwarded.
Script-Cookie = "CGI-SET-COOKIE" token SIP-Version
This action line causes the server to store a script cookie, passed
as a token in the action line. Subsequent script invocations for
messages within the same transaction carry the token in a meta-
header. The script can alter the value of the cookie by subsequent
script cookie actions. This alteration will take affect for all
subsequent script invocations.
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CGI-Again = "CGI-AGAIN" ("yes" | "no") SIP-Version
This action line determines whether the script will be invoked for
subsequent requests and responses for this transaction. If the
parameter "yes" is given to this action, the script will be executed
again when the next message arrives. If the parameter is "no," or
this action is not specified, the script will not be executed again,
and the server will perform its default action for all subsequent
messages.
If none of the actions CGI-PROXY-REQUEST, CGI-FORWARD-RESPONSE, or a
new response are performed -- that is to say, the script outputs only
CGI-AGAIN, CGI-SET-COOKIE, or nothing -- the script performs its
default action. The default action to take depends on the event
which triggered the script:
Request received: When the request is first received, the
default action of the server is to check whether the
domain of the server matches the domain of the Request-
URI. If it does not, the request is proxied to the
request in the Request-URI. Otherwise, the server checks
its registration database against the request, and either
proxies or redirects the request based on the action
specified by the user agent in the registration.
Proxied response received: If a response is received to a
proxied request, the server forwards the response towards
the caller if the response was a 200 or 600 class
response, and sends a CANCEL on all pending branches. If
the response was 100 class, the state machinery for that
branch is updated, and the response is proxied upstream
towards the caller unless the it was a 100 response, not
some other 1xx. For 300, 400, and 500 class responses,
an ACK is sent, and the response is forwarded upstream
towards the caller if all other branches have terminated,
and the response is the best received so far. If not all
branches have terminated, the server does nothing. If
all branches have terminated, but this response is not
the best, the best is forwarded upstream. This is the
basic algorithm outlined in the SIP specification.
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CGI header fields syntactically resemble SIP header fields, but their
names all begin with the string "CGI-". The SIP server MUST strip
all CGI header fields from any message before sending it, including
those it does not recognize.
CGI header fields have the generic syntax specified in section 6.6 of
the SIP/2.0 specification [2]. The field-name is not case sensitive;
the field value MUST conform to the grammar of that specific field in
the specification where it is defined.
Request-Token = "CGI-Request-Token" ":" token
To assist in matching responses to proxied requests, the script can
place a CGI-Request-Token CGI header in a CGI-PROXY-REQUEST or new
request. This header contains a token, opaque to the server. When a
response to this request arrives, the token is passed back to the
script as a meta-header.
This allows scripts to "fork" a proxy request, and
correlate which response corresponds to which branch of the
request.
Remove = "CGI-Remove" ":" 1#field-name
The CGI-Remove header allows the script to remove SIP headers from
the outgoing request or response. The value of this header is a
comma-separated list of SIP headers which should be removed before
sending out the message.
A script MAY specify headers which are not in the request; the server
SHOULD silently ignore these. A script SHOULD NOT both specify a SIP
header in its output and also list that header in a CGI-Remove
header; the result of doing this is undefined.
If a CGI script specifies an Expires header field along with CGI-
PROXY-REQUEST, the SIP server SHOULD track the expiration timeout
locally as well as sending the message to the remote server. When
the timeout expires, the server SHOULD generate a "408 Request
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Timeout" response. The timeout response SHOULD be handled as
specified in section 5.8. At the time the request is timed out, the
server SHOULD also transmit CANCEL messages for the request.
This allows a SIP CGI script in a proxy server to implement
services like "Call Forward No Answer" to trigger after a
user-determined time, even if the remote user-agent server
is not responding or does not properly handle the Expires
header field.
In a proxy environment, locally-generated responses such as "408
Request Timeout" SHOULD be sent to the CGI script in the same manner
as received messages are. However, messages which merely report a
problem with a message, such as "400 Bad Request", SHOULD NOT be.
This is the other half of the requirements for the
implementation of the "Call Forward No Answer" service,
along with the local handling of the Expires header.
The specific semantics of a SIP CGI script which is triggered by a
REGISTER request are somewhat different than that of those triggered
by call-related requests; however, allowing user control of
registration may in some cases be useful. The two specific actions
for REGISTER that need to be discussed are the response "200" and the
default action. In the former case, the server SHOULD assume that
the CGI script is handling the registration internally, and SHOULD
NOT add the registration to its internal registration database; in
the latter case, the server SHOULD add the registration to its own
database. The server also SHOULD NOT add the registration if a 3xx,
4xx, 5xx, or 6xx status was returned, or if the registration request
was proxied to another location.
SIP CGI servers SHOULD execute scripts when a CANCEL message is
received. The script SHOULD clean up any state it has for the
transaction as quickly as possible.
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When a CANCEL is received at a server for an existing transaction,
the server SHOULD send a 200 OK response to the cancel and cancel all
currently outstanding branches. The transmission of the script on a
CANCEL message is purely advisory, and the script SHOULD NOT perform
any actions in response to it.
Under normal circumstances, if the server receives an ACK, the script
is not re-executed. If the ACK is destined for the proxy
(acknowledging a 300, 400, 500, or 600 response), the ACK causes
response retransmissions to cease. If the ACK is for a 200 response
forwarded from a downstream server, the ACK is proxied downstream.
However, if the script generated its own 200 response to an INVITE
request, the script SHOULD be re-executed with the ACK message. This
is necessary in cases where the script is causing the proxy to act as
a UAS. ACK messages can contain bodies, and would therefore be
useful to the script.
When the server receives a non-200 final response to an INVITE
request, it SHOULD generate an ACK on its own, and not depend on the
script to do so. There is no way in SIP CGI 1.1 to override this
behavior. However, since the server will not generate an ACK for 200
responses to INVITE, a script causing the server to act as a UAC MUST
generate ACK's for them.
6 System Specifications
The implementation of SIP CGI on a Unix operating system platform
SHOULD use environment variables as the mechanism of providing
request metadata to CGI scripts.
For Unix compatible operating systems, the following are defined:
Environment variables: These are accessed by the C library
routine getenv.
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The current working directory: The current working directory for
the script SHOULD be set to the directory containing the
script.
Character set: The US-ASCII character set is used for the
definition of environment variable names and header field
names; the newline (NL) sequence is LF; servers SHOULD also
accept CR LF as a newline.
The implementation of SIP CGI on 32-bit Microsoft Windows system
platforms (Windows 95, 98, NT, and 2000) SHOULD use environment
variables as the mechanism of providing request metadata to CGI
scripts.
For Microsoft Windows, the following are defined:
Environment variables: These are accessed by the C library
routine getenv.
The current working directory: The current working directory for
the script SHOULD be set to the directory containing the
script.
Character set: The US-ASCII character set is used for the
definition of environment variable names and header field
names; the newline (NL) sequence is CR LF; servers SHOULD
also accept LF as a newline.
7 Security Considerations
CGI scripts are able to initiate arbitrary SIP transactions, or to
produce spoofed responses of any sort. This protocol does not
attempt to restrict the actions CGI scripts can take. Server
administrators MUST consider CGI scripts to be as security-sensitive
as their SIP server itself, and perform equivalent levels of security
review before installing them.
CGI scripts must be careful not to interfere with authentication. In
particular, adding or removing header fields that are below the
Authorization header will cause the message to fail authentication at
the user agent.
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When a SIP request is encrypted, the headers which are in the clear
are passed to the server according to this specification. The
encrypted portion of the request is passed to the script as a body.
Any SIP headers output by the script will be added to the message.
However, scripts should be aware that these may be discarded if they
also exist within the encrypted portion.
Some SIP header fields may carry sensitive information which the
server SHOULD NOT pass on to the script unless explicitly configured
to do so. For example, if the server protects the script using the
Basic authentication scheme, then the client will send an
Authorization header field containing a username and password. If
the server, rather than the script, validates this information then
the password SHOULD NOT be passed on to the script via the
HTTP_AUTHORIZATION metavariable.
The most common implementation of CGI invokes the script as a child
process using the same user and group as the server process. It
SHOULD therefore be ensured that the script cannot interfere with the
server process, its configuration, or documents.
If the script is executed by calling a function linked in to the
server software (either at compile-time or run-time) then precautions
SHOULD be taken to protect the core memory of the server, or to
ensure that untrusted code cannot be executed.
This specification places no limits on the length of entity bodies
presented to the script. Scripts SHOULD NOT assume that statically
allocated buffers of any size are sufficient to contain the entire
submission at one time. Use of a fixed length buffer without careful
overflow checking may result in an attacker exploiting `stack-
smashing' or `stack-overflow' vulnerabilities of the operating
system. Scripts may spool large submissions to disk or other
buffering media, but a rapid succession of large submissions may
result in denial of service conditions. If the CONTENT_LENGTH of an
entity-body is larger than resource considerations allow, scripts
SHOULD respond with `413 Request Entity Too Large.'
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8 Acknowledgements
This work draws extremely heavily upon the HTTP CGI specification
[1]; approximately half the text of the specification section is
taken from that document.
9 Authors' Addresses
Jonathan Lennox
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
USA
EMail: lennox@cs.columbia.edu
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Ave.
First Floor
East Hanover, NJ 07936
EMail: jdrosen@dynamicsoft.com
Henning Schulzrinne
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
USA
EMail: schulzrinne@cs.columbia.edu
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10 Bibliography
[1] http://hoohoo.ncsa.uiuc.edu/cgi/interface.html
[2] Handley, M, Schulzrinne, H., Schooler, E. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999.
[3] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 10, RFC 822, August 1982.
[4] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", BCP 14, RFC 2119, March 1997.
[5] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
[6] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, November
1996.
[7] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[8] St. Johns, M., "Identification Protocol", RFC 1413, January
1993.
[9] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
Lennox, et al. Informational [Page 34]
RFC 3050 CGI for SIP January 2001
11 Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
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The limited permissions granted above are perpetual and will not be
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