One document matched: draft-lennox-sip-reg-payload-00.txt
Internet Engineering Task Force Individual Submission
Internet Draft Lennox/Schulzrinne
draft-lennox-sip-reg-payload-00.txt Columbia University
March 6, 2000
Expires: September 2000
Transporting User Control Information in SIP REGISTER Payloads
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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Abstract
Several newly developed languages and interfaces, such as the CPL and
SIP CGI, allow users or administrators to specify how Internet
telephony servers should process calls. There needs to be a method of
transporting scripts for such languages between a client and a
server. This document proposes using the payload of SIP REGISTER
messages, and their responses, as one method to transport them.
1 Introduction
Several newly developed languages and interfaces, such as the CPL [1]
and SIP CGI [2] allow users or administrators to specify how Internet
telephony servers should process calls. Scripts typically can be
created on a client, but executed on an Internet telephony server.
There therefore needs to be a method of transporting these scripts
from a client to a server, and of retrieving them from the server so
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the client can know the current status or modify the script. This
method should integrate cleanly with the existing infrastructure of
Internet telephony, without requiring significant additional protocol
traffic or complexity in either a client or a server.
This document proposes using the payload of SIP [3] REGISTER
messages, and their responses, as the media to transport these
scripts to SIP registration servers alongside the user's
registration. Since clients typically will need to register anyway,
and servers will need to have registrars to process the clients'
registrations, this technique does not impose much additional
overhead on servers and clients.
This technique is not appropriate for all environments -- most
obviously, it is not useful for H.323 [4] servers -- and we do not
anticipate that it will be the only such transport mechanism
developed. Other protocols considered have included transporting
scripts over LDAP [5], ACAP [6], or HTTP file upload [7], or
transport mechanisms developed from scratch.
2 Conventions Of This Document
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 [8] 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 the specification. They
are not normative to the specification of the protocol.
3 Transport Details
To upload a script, the registration client places the script in the
body of the SIP REGISTER request. Bodies of SIP requests are
described in [3]. The Content-Type header field is set to the media
type of the submitted script. Currently, we expect to register
application/cpl for CPL scripts; however, this document is not a MIME
registration for this type. Clients SHOULD upload SIP CGI scripts as
an appropriate media type for the language the script is written in
(for example, application/x-perl), or application/octet-stream if no
such media type exists or is known. Registrars MAY perform validation
on the media types if they know certain types of scripts cannot be
executed on their servers, but SHOULD be permissive about unknown or
ambiguous media types for SIP CGI scripts.
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Which types of SIP CGI scripts can be sucessfully run on a
server depends on the server's environment, including which
scripting languages are installed on it. It is possible
that the user has more knowledge of this environment than
the server.
Script uploads MUST also be accompanied by two new SIP headers:
Content-Purpose and Content-Action. The grammar of these headers is
as follows:
Content-Purpose = "Content-Purpose" ":" purpose
*( ";" extension-attribute )
purpose = "script" | "sip-cgi" | token
Content-Action = "Content-Action" ":" action
*( ";" extension-attribute )
action = "add" | "delete" | token
The grammar symbols "token" and "extension-attribute" are defined in
RFC 2543 [3].
The Content-Purpose header serves to define the purpose of the
specified content. Currently, two purposes are defined: "script"
refers to CPL scripts and other, future scripting environments whose
calling conventions can be uniquely determined by their media type;
"sip-cgi" refers to SIP CGI scripts, which can be any media type
executable on the server platform. Additional purposes may be
registered with IANA.
We anticipate that this mechanism in this specification can
also be used for purposes such as users' speed-dial lists
or device configuration files, and that new purposes would
be registered for these.
A similar syntax has been proposed for including multiple
bodies in SIP INVITE requests, containing information such
as billing information, ISUP interworking, and so forth, in
addition to the session description. Christian Huitema
proposed a specific solution [9] some time ago, but working
group consensus was that a more general "Content-" family
header would be more appropriate. However, no further
Internet-Drafts seem to have come of this. If or when such
progress is made, it is our intention that the Content-
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Purpose header of this draft should be unified with that
header.
The Content-Action header is used when uploading scripts, to specify
what the server should do with the script uploaded. If a non-zero-
length script is specified, the action "add" MUST be given. The
action "delete" MUST only be used with a zero-length script.
A script registrar's normal behavior is to enter the script in its
database, as specified in section 4. However, if a zero-length script
is submitted with the action "delete," any existing scripts are
instead deleted from the database.
Note that, therefore, a zero-length script and the absence
of any script are quite distinct phenomena, and both are
legal.
If multipart MIME types [10] are used, these headers MUST be included
in the MIME part headers, not in the general SIP headers.
To inform a client of what types of scripts it supports, a server
SHOULD send the media types of its supported scripts in Accept header
fields in the response to any successful OPTIONS request directed at
the server's registrar address. It MAY also include this in the
response to a successful REGISTER request, and of course SHOULD
include it in any response which rejected a registration on the
grounds of an unsupported media type (as specified in section 7.4.15
of RFC 2543 [3].
Note: this is against the strict wording of the SIP
specification, which says that Accept headers are only
allowed in requests or 415 (Unsupported Media Type)
responses. However, it is always legal to include a header
in any request or response, as clients which do not
understand it in a given context simply ignore it. The
revised SIP specification (not yet published as an
Internet-Draft, but available from
http://www.cs.columbia.edu/~ hgs/sip/) is expected to fix
this problem.
Question: a Accept-Purpose header has also been proposed to
list the content purposes the server supports. Is this a
good idea?
In a successful response to any REGISTER request, whether or not a
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script payload was included, the server SHOULD return the currently
specified script in the response body, with its type specified in
Content-Type, and its purpose in the Content-Purpose, unless the
REGISTER request included a set of Accept headers which did not
include the type of the registration script. The reply sent by the
server SHOULD NOT include Content-Action headers, as the server is
not requesting that the client perform any actions. The server
SHOULD NOT return the currently registered script if response to the
registration request was an error condition.
Allowing the client to restrict transmitted scripts by
media type allows clients connected by a low-bandwidth
network avoid downloading lengthy scripts.
The server MAY perform validation on scripts at the time they are
uploaded to the server. If the script is not valid, the server SHOULD
return a 400-class error to the registration request indicating the
problem. It MAY include in the body of the response an explanation
of why the script was considered invalid, if the registration
included an Accept header with an appropriate media type for such an
explanation (such as text/html or text/plain)
When a script with the same purpose as an existing script is
uploaded, the script is replaced in the server. Which script applies
to calls in progress at the time the script was changed is not
defined by this document, but MAY be specified by specifications of
script languages. If the current or new script affects the handling
of REGISTER requests, the registration is handled entirely by the
existing script; the new script does not take effect until the
registration process is complete. Scripts with different purposes
are stored and deleted independently. However, a server MAY choose
not to execute some scripts if scripts with another purpose are
present, for instance only executing one of a CPL and SIP CGI script.
To delete a script, a client sends a REGISTER message with its
Content-Purpose header set to the appropriate value, a Content-Length
of 0, and a Content-Action of "delete". If there is no script
defined with the specified purpose, this message does nothing. When
a script is deleted, the server SHOULD return to its default call
handling behavior for subsequent calls, just as if no script had ever
been uploaded. As for changing scripts, the effect of deleting
scripts on calls currently in progress is not defined by this
specification.
Question: should the technique described in this
specification have Require or Supported headers defined?
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4 Persistence Model
Registrations in SIP are normally transient -- the data in the
Contact header fields last only for the length of time specified in
the registration's Expires header, and clients must refresh their
registrations periodically.
In contrast, scripts sent to registration servers using the method
described in this document are persistent -- they remain in the
server until replaced or deleted, and they do not need to be
refreshed. Servers SHOULD therefore store uploaded scripts in non-
volatile storage so they persist through server restarts or failures.
Clients SHOULD only upload scripts when they are explicitly requested
to, and SHOULD NOT transmit their scripts in every registration
request.
The model of standard SIP registrations is that each client
registers itself; if a location changes or hosts die, old
registrations naturally time out. Since a user can be
simultaneously registered from many locations, several
clients re-registering periodically present no conflicts.
The model of scripts is quite different. A user only has one script
(or at least only of a given type) at a time, so if clients
periodically re-uploaded scripts, two clients with different
specified scripts would cause "script flapping," as the behavior
specified in the server changed frequently, with unpredictable and
probably surprising behavior. Moreover, one of the most important
purposes of scripts is to control the processing of a user's requests
when he or she is not registered from any location; if scripts timed
out and had to be refreshed, this goal could not be accomplished.
5 Examples
The first example shows a user uploading a simple call-filtering SIP
CGI script written in Perl to his server. Note that he is
transmitting both a contact address, which persists only for 30
minutes, the time specified by the Expires header, and a script,
which persists indefinitely. This allows him subsequently to
register new contact addresses and have his script apply equally to
them. (See [2] for an explanation of SIP CGI as used in the script.)
The use of Basic authorization here is for the purposes of the
example only; in actual practice much more robust authentication
SHOULD be used. See section 7.
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REGISTER sip:@sip.example.com SIP/2.0
From: Joe User <sip:joe@example.com>
To: "J. User" <sip:joe@example.com>
CSeq: 18 REGISTER
Expires: 1800
Call-ID: 39485832@joespc.example.com
Contact: sip:joe@joespc.example.com
Accept: application/x-perl, application/sdp, text/html
Authorization: Basic am9lOnBhc3N3b3JkAFBX
Content-Type: application/x-perl
Content-Length: 137
Content-Purpose: sip-cgi
Content-Action: add
#!/usr/bin/perl
if ($ENV{HTTP_FROM} =~ /telemarketers.com/) {
print "SIP/2.0 603 Go awayn"
} else {
exit(0); # Default action
}
In the second example, a few minutes later, the user registers a new
contact address, but does not change his script. In the response to
the registration, the server reminds him of his contact addresses and
his current script.
His client sends this request:
REGISTER sip:@sip.example.com SIP/2.0
From: Joe User <sip:joe@example.com>
To: "J. User" <sip:joe@example.com>
CSeq: 19 REGISTER
Expires: 1800
Call-ID: 39485832@joespc.example.com
Contact: sip:joe@joeshome.example.com
Accept: application/x-perl, application/sdp, text/html
Authorization: Basic am9lOnBhc3N3b3JkAFBX
Content-Length: 0
And the server replies with this response:
SIP/2.0 200 OK
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From: Joe User <sip:joe@example.com>
To: "J. User" <sip:joe@example.com>
CSeq: 19 REGISTER
Contact: sip:joe@joespc.example.com
Contact: sip:joe@joeshome.example.com
Accept: application/cpl, */*
Content-Type: application/x-perl
Content-Length: 137
Content-Purpose: sip-cgi
#!/usr/bin/perl
if ($ENV{HTTP_FROM} =~ /telemarketers.com/) {
print "SIP/2.0 603 Go awayn"
} else {
exit(0); # Default action
}
Finally, the user decides to eliminate his script, and the server
responds in the same manner as it would respond to an ordinary
registration, as though no script had ever been uploaded:
REGISTER sip:@sip.example.com SIP/2.0
From: Joe User <sip:joe@example.com>
To: "J. User" <sip:joe@example.com>
CSeq: 20 REGISTER
Call-ID: 39485832@joespc.example.com
Contact: sip:joe@joeshome.example.com
Authorization: Basic am9lOnBhc3N3b3JkAFBX
Accept: application/x-perl, application/sdp, text/html
Content-Length: 0
Content-Purpose: script
Content-Action: delete
SIP/2.0 200 OK
From: Joe User <sip:joe@example.com>
To: "J. User" <sip:joe@example.com>
CSeq: 20 REGISTER
Contact: sip:joe@joespc.example.com
Contact: sip:joe@joeshome.example.com
Accept: application/x-perl, application/cpl, */*
Content-Length: 0
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6 Usage notes
Because scripts can be long, clients which upload scripts, or which
present an Allow header which could cause scripts to be returned,
SHOULD send their REGISTER messages over TCP rather than UDP.
7 Security Considerations
Since the scripts transported by this mechanism control how a server
directs private information intended for a user, the server MUST
reject all un-authenticated attempts to submit a script, and SHOULD
require that the authentication method used verifies the integrity of
the submitted script; for example, by having the entire request,
including its body, signed with SIP's PGP authentication method.
8 IANA Considerations
The Content-Purpose header in section 3 can have additional purposes
defined.
TODO: need specific registration procedure. TBD if
consensus is that this approach is the right way to do
things.
9 Changes from earier versions
This document was originally published as draft-iptel-sip-reg-
payload-00, but the consensus of the IPTel working group was that
this should not be a work item of that group.
9.1 Changes from IPTel draft -00
The changebars in the Postscript and PDF versions of this document
indicate significant changes from this version.
o Added Content-Purpose and Content-Action headers.
o Changed the procedure by which scripts are deleted.
o Eliminated the pseudo-media-type application/sip-cgi, as it is
counter to the spirit of MIME. Instead, established that SIP
CGI scripts can be any media type.
o Added "Conventions," "Usage Notes," and "IANA Considerations"
sections.
o Updated examples to use the syntax of the current version of
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SIP CGI.
o Updated references to refer to the latest versions of all
documents.
10 Authors' Addresses
Jonathan Lennox
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
USA
electronic mail: lennox@cs.columbia.edu
Henning Schulzrinne
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
USA
electronic mail: schulzrinne@cs.columbia.edu
11 Bibliography
[1] J. Lennox and H. Schulzrinne, "CPL: a language for user control
of internet telephony services," Internet Draft, Internet Engineering
Task Force, Mar. 1999. Work in progress.
[2] J. Lennox, J. Rosenberg, and H. Schulzrinne, "Common gateway
interface for SIP," Internet Draft, Internet Engineering Task Force,
May 1999. Work in progress.
[3] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
session initiation protocol," Request for Comments (Proposed
Standard) 2543, Internet Engineering Task Force, Mar. 1999.
[4] International Telecommunication Union, "Packet based multimedia
communication systems," Recommendation H.323, Telecommunication
Standardization Sector of ITU, Geneva, Switzerland, Feb. 1998.
[5] M. Wahl, T. Howes, and S. Kille, "Lightweight directory access
protocol (v3)," Request for Comments (Proposed Standard) 2251,
Internet Engineering Task Force, Dec. 1997.
[6] C. Newman and J. G. Myers, "ACAP -- application configuration
access protocol," Request for Comments (Proposed Standard) 2244,
Internet Engineering Task Force, Nov. 1997.
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[7] E. Nebel and L. Masinter, "Form-based file upload in HTML,"
Request for Comments (Experimental) 1867, Internet Engineering Task
Force, Nov. 1995.
[8] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," Request for Comments (Best Current Practice) 2119, Internet
Engineering Task Force, Mar. 1997.
[9] C. Huitema, "The multipart/sip-id media type," Internet Draft,
Internet Engineering Task Force, Feb. 1999. Work in progress.
[10] N. Freed and N. Borenstein, "Multipurpose internet mail
extensions (MIME) part two: Media types," Request for Comments (Draft
Standard) 2046, Internet Engineering Task Force, Nov. 1996.
Full Copyright Statement
Copyright (c) The Internet Society (2000). All Rights Reserved.
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Table of Contents
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1 Introduction ........................................ 1
2 Conventions Of This Document ........................ 2
3 Transport Details ................................... 2
4 Persistence Model ................................... 6
5 Examples ............................................ 6
6 Usage notes ......................................... 9
7 Security Considerations ............................. 9
8 IANA Considerations ................................. 9
9 Changes from earier versions ........................ 9
9.1 Changes from IPTel draft -00 ........................ 9
10 Authors' Addresses .................................. 10
11 Bibliography ........................................ 10
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