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Versions: 00 01
D. Petrie
Internet Draft Pingtel Corp.
Document: draft-petrie-sip-config-framework-00.txt
Expires: September 2001 March 2001
A Framework for SIP User Agent Configuration
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
Task Force (IETF), its areas, and its working groups. Note that
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reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
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Abstract
This document defines the application of a set of protocols for
configuring a SIP user agent. The SIP user agent must discover how
and from where to retrieve its initial configuration and be notified
of changes and updates which impact its configuration. The
objective is to define a means for automatically configuring a user
agent such that it can be functional without user or administrative
intervention. The framework for discovery, delivery, notification
and updates of user agent configuration is defined here. This
framework is also intended to ease ongoing administration,
configuration and upgrading of large scale deployments of SIP user
agents. The contents and format of the configuration data to be
defined is outside the scope of this document.
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Table of Contents
Status of this Memo................................................1
Abstract...........................................................1
1 Overview.......................................................3
2 Conventions used in this document..............................4
3 Discovery......................................................4
3.1 DHCP Option..................................................5
3.2 DNS SRV......................................................5
3.3 DNS..........................................................5
3.4 Multicast....................................................5
3.5 Manually Provisioned.........................................5
4 Enrollment and Change Notification.............................6
4.1 Header Field Definitions.....................................7
4.1.1 Config-Allow................................................7
4.1.2 Config-Require..............................................7
4.1.3 Config-Expires..............................................7
4.2 SUBSCRIBE....................................................8
4.2.1 Additional From Field Parameters............................9
4.3 NOTIFY.......................................................9
4.3.1 NOTIFY Body Content Format.................................10
5 Configuration Retrieval.......................................11
6 Configuration Upload..........................................11
7 Examples......................................................12
7.1 Example Message Flows.......................................12
7.2 Example Messages............................................14
8 Security Considerations.......................................17
9 Open Issues...................................................18
10 References....................................................19
11 Author's Addresses............................................20
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1 Overview
This document defines a framework which allows SIP user agents (UA)
to automatically:
- discover a configuration server (Discovery)
- enroll with the configuration server (Enrollment)
- retrieve configuration data (Configuration Retrieval)
- receive notification of configuration changes (Change
Notification)
- upload configuration data changes back up to the server
(Configuration Upload)
The content and format of the data is not defined in this document.
It is to be defined in configuration data profile(s) in other
document(s). The goal of this framework is to satisfy the
requirements defined in [10] and [11] excluding the requirements
which pertain to configuration data profile content and format.
Discovery is the process by which a UA SHOULD find the address and
port at which it SHOULD enroll with the configuration server. As
there is no single discovery mechanism which will work in all
network environments, a number of discovery mechanisms are defined
with a prescribed order in which the UA SHOULD try them until one
succeeds.
Enrollment is the process by which a UA SHOULD make itself known to
the configuration server. In enrolling the UA MUST provide identity
information, a named list of requested configuration data profiles
and supported protocols for configuration retrieval. It SHOULD also
SUBSCRIBE to a mechanism for notification of configuration changes.
As a result of enrollment the UA receives a URL for each of the
configuration data profiles that the configuration server is able to
provide.
Configuration Retrieval is the process of retrieving the content for
each of the configuration data profiles the UA requested.
Change Notification is the process by which the configuration server
notifies the UA that the content of one or more of the configuration
data profiles has changed. Subsequently the UA SHOULD retrieve the
data profile from the specified URL upon receipt of the change
notification.
Configuration Upload is the process by which a UA or other entity
pushes a change to a configuration data profile back up to the
configuration server.
Today all SIP UA vendors use proprietary means of delivering
configuration to the UA. This configuration framework is intended
to enable a first phase migration to a standard means of configuring
SIP user agents. It is expected that UA vendors should be able to
use this configuration framework as a means of delivering their
existing proprietary configuration data profiles (i.e. using their
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existing proprietary binary or text formats). This in itself is a
tremendous advantage in that a SIP environment can use a single
configuration server to deliver configuration data to UAs from
multiple vendors. Follow-on standardization activities can: 1)
define a standard format (e.g. XML or name-value pairs [8]) and 2)
specify the content (i.e. name the configuration parameters) of the
configuration data profiles.
2 Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119 [1].
The syntax and semantics used here extend those defined in SIP (RFC
2543) [6]. SIP is described in an augmented Backus-Naur form (ABNF).
See [6, section C] for an overview of ABNF.
3 Discovery
The first time a UA is plugged in it does not know the address or
port at which to enroll with the local configuration server. It
must discover this address and port. A UA SHOULD support all of the
listed discovery mechanisms. It MUST support at least one of them.
Once the UA has discovered the address and port and has successfully
enrolled with the configuration server, the UA SHOULD cache the
address and port to avoid the need to re-discover the configuration
server. However if enrollment, configuration retrieval or
configuration upload fails at any time, the UA SHOULD apply the
discovery and enrollment process again. This provides a means for
configuration server fail over and load balancing.
The UA SHOULD use the following mechanisms to discover the host
address and port at which it SHOULD enroll with the configuration
server. Each mechanism should be tried in the following order until
an address and port is provided which results in successful
enrollment (i.e. the server responds with a successful 2xx class
response):
- DHCP site-specific option [1]
- DNS SRV
- DNS A record
- Multicast
- Manual provisioning
The rationale for this order follows. Assuming that most UAs are
going to use DHCP for IP configuration anyway, using a DHCP option
is the least costly in terms of lookup time (i.e. no additional
messages are required). Hence DHCP is first. DNS SRV allows the
more flexibility than DNS A records. Hence DNS SRV is tried before
DNS A records. Multicast is used last of the automated discovery
mechanisms as it is the most restricted in terms of network
environments that support it. Multicast is included, even though
the applicable environments are restricted, as it is the only
mechanism that can be used without the support of the local network
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administrator (The phone administrator and the network administrator
are often different people and perhaps in different departments.)
The UA implementer MAY provide the user or administrator with the
means to change the order in which these mechanisms are tried.
However by default without user interaction it SHOULD use the order
listed above.
3.1 DHCP Option
It is likely that most UAs in an environment of any significant
number will use DHCP for IP configuration. DHCP becomes a
convenient means to discover the configuration server. In the same
DHCP request for basic IP configuration, the UA can add the site-
specific option [TBD] [1] to the options field. This indicates a
request for the configuration server address and port. If the
configuration server address and port is not returned in the DHCP
response or the server does not respond with a successful 2xx class
response, the next discovery mechanism is attempted.
[site-specific DHCP option (i.e. > 128)?]
or
[DHCP Option for SIP Servers?]
3.2 DNS SRV
Using the service identifier sipuaconfig the DNS SRV records [5]
are requested for the local domain for the protocols (i.e. UDP
and/or TCP) that the UA supports. The UA tries to enroll using the
search order as prescribed in RFC 2782 [5]. If none of the servers
respond with a successful 2xx class response (or none are returned
in the SRV records) the next discovery mechanism is attempted.
3.3 DNS
The UA SHALL try a DNS A record lookup on the host name
sipuaconfig. If the server does not respond with a successful 2xx
class response, the next discovery mechanism is attempted.
3.4 Multicast
The enrollment request is sent to the multicast address for SIP
registration [6] "sip.mcast.net" (224.0.1.75). If a server does not
respond with a successful 2xx class response to the enrollment
request, the next discovery mechanism is attempted.
3.5 Manually Provisioned
The UA SHOULD let the user (or administrator) know if the automatic
discovery has failed and allow the user or administrator to manually
(or perhaps using some other out of band means e.g. beam, smart
card, etc.) enter the configuration server address and port to be
used for enrollment.
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4 Enrollment and Change Notification
The enrollment and configuration change notification are paired
together and provided via the SIP SUBSCRIBE/NOTIFY framework [7].
This document defines the profile on top of the SUBSCRIBE/NOTIFY
framework [7] for this purpose.
UA enrollment with the configuration server is accomplished via the
SUBSCRIBE request. A UA MUST enroll with the configuration server
prior to retrieving configuration data profiles. As part of the
enrollment the UA MUST identify itself, its configuration retrieval
protocol capabilities and configuration data profile requirements.
The configuration server may use this information to decide how to
allocate resources (e.g. load balancing) to support the UA for its
specific configuration retrieval needs. The configuration server
may also use the UA enrollment event as the trigger to generate a
new set of configuration data for the specific UA (e.g. based upon
provisioned defaults and configuration profile context knowledge for
the environment). This would allow the configuration server to
provide configuration data for a new UA without previously
provisioning the specific UA on the server.
Configuration Change Notification is communicated to the UA via a
NOTIFY request from the configuration server. The NOTIFY request is
used by the configuration server to convey the URLs that the UA MUST
use to retrieve its requested configuration data profiles. The
NOTIFY is used immediately after enrollment. It MAY be subsequently
used by the configuration server to identify the list of
configuration data profile URLs which have changed (i.e. change
notification).
The SUBSCRIBE request for enrollment is sent to the address(es)
identified in the discovery process until the first successful 2xx
class response is received. As part of the binding of the
SUBSCRIBE/NOTIFY framework this document defines a new Event token:
Config-Event. The Event header field is mandatory in SUBSCRIBE
and NOTIFY requests and MUST contain this token value when used for
the purpose of enrollment and configuration change notification.
[Does Config-Event need to be registered with IANA?]
If enrollment fails (i.e. no 2xx response to SUBSCRIBE), the UA
SHOULD re-discover the configuration server address and port as
described in section 3.
The following new header fields are defined for use in SUBSCRIBE and
NOTIFY requests for the purpose of enrollment and configuration
change notification:
The keys used the following table:
R request
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r response
m mandatory
o optional
- - not applicable
Header Where SUBSCRIBE NOTIFY
------ ----- --------- ------
Config-Allow R m -
Config-Require R m -
Config-Expires R - o
4.1 Header Field Definitions
4.1.1 Config-Allow
The Config-Allow header field is used by the UA in the enrollment
request (SUBSCRIBE) to list the protocols that it is capable of
using to retrieve configuration data. The configuration server MUST
adhere to the protocol capabilities of the UA when providing the
list of URLs for the configuration profiles in the NOTIFY request.
Syntax:
Config-Allow = "Config-Allow" ":" 1#config-protocol
config-protocol = tftp | http | https | token
4.1.2 Config-Require
The Config-Require header field contains the names of all of the
configuration data profiles that the UA requires. The name(s) of
the configuration profiles are to be defined in a future document(s)
specifying the content and format of the specific profile.
Syntax:
Config-Require = Config-Require : 1#config-profile-name
config-profile-name = token
[experimental: x-
?]
[IANA ?]
4.1.3 Config-Expires
The optional Config-Expires header field defines the lease length of
the configuration data. If Config-Expires was present in the last
NOTIFY received from the configuration server and the UA has not
received a notification from the configuration server within this
period of time, the UA SHOULD re-enroll by sending a new SUBSCRIBE
message to the configuration server. If the enrollment fails, the
UA SHOULD re-discover the configuration server using the mechanisms
described in section 3. The configuration server SHOULD send a
NOTIFY before the lease expires with the event Config-Event, a
renewed lease length in the Config-Expires header field and the
complete list of configuration data profile URLs in the request
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body for the UAs configuration data. The configuration data
profile URLs SHOULD have the same sequence numbers if the content
has not changed. The sequence numbers MUST be different for
profiles whose content has changed. The absence of the Config-
Expires header field in the lease renewal indicates an indefinite
expiration.
Note: the Config-Expires header field sets a lease that the UA
observes to determine when its configuration is stale. This lease
is renewed with every NOTIFY message from the configuration server.
The Expires header field in the SUBSCRIBE request describes the
duration that the configuration server will continue to send change
notifications to the UA. This is renewed with every SUBSCRIBE
request from the UA.
Syntax:
Config-Expires = Config-Expires : delta-seconds
4.2 SUBSCRIBE
The SUBSCRIBE request is used by the UA to enroll in the
configuration domain of the configuration server. It uniquely
identifies the UA with vendor, model and serial number information.
The UA also MUST specify its capabilities for configuration
retrieval as well as the configuration data profiles that it
requires. That is the UA MUST include the Config-Allow and Config-
Require header fields and each MUST contain at least one token. The
configuration server MUST not send an error if it is not able to
provide all of the configuration data profiles listed in the
SUBSCRIBE request Config-Require header field. The configuration
server SHOULD provide the configuration data profile that it is able
to or desires (see example at the end of section 4.3) to deliver to
the UA. If the configuration server sends a 301 Moved Permanently
response to the enrollment SUBSCRIBE, the UA SHOULD cache the URL
contained in the response Contact header field in place of the
address and port found during discovery for future enrollment.
The configuration server MAY use the enrollment (SUBSCRIBE request)
as the stimulus to generate a new instance of a configuration data
profile unique to the UA. Alternately the configuration server MAY
be provisioned ahead of time to know about new UAs and their
specific configuration data content (for example based upon serial
number, MAC address).
[Request URI should not contain a user id? The user may not be
known yet.]
[ What happens when the config server receives multiple SUBSCRIBE
requests from the same UA but for different list of profiles. Does
the last request supercede all previous ones?]
[ What happens when the config server receives multiple SUBSCRIBE
requests from the same IP address but for different devices? This
might happen if some entity is acting as a proxy for a bunch of
other devices. It might also happen if the IP address for a UA gets
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reused for some other UA (the DHCP lease timeout may be much shorter
than the SUBSCRIBE/NOTIFY lease timeouts). The SUBSCRIBE lease
SHOULD not exceed the DHCP lease? The UA SHOULD reenroll if its IP
address changes?]
4.2.1 Additional From Field Parameters
In the enrollment and configuration change notification messages
(i.e. SUBSCRIBE and NOTIFY requests and responses) the SIP-URL [6]
MUST not contain userinfo if the default UA user is not known (e.g.
first time startup of new UA out of the box).
The following additional From field parameters are defined for the
purpose of identifying the UA device:
Vendor a token used to identify the UA vendor name
Model a token used to identify the UA hardware/software model
Version a token used to identify the firmware/software version
currently installed on the UA
Serial the token used to identify the serial number for the UA
Mac the token used to identify the MAC address in hex for the UA
From RFC 2543 [6] the From header field syntax is extended to
include:
from-param = tag-param | generic-param | device-param
device-param = vendor-parm | model-parm | version-parm |
serial-parm | mac-parm
vendor-parm = Vendor = token
model-parm = Model = token
version-parm = Version = token
serial-parm = Serial = token
mac-parm = Mac = token
4.3 NOTIFY
The NOTIFY message is sent by the configuration server to convey the
URLs at which the UA can retrieve the requested configuration data
profiles. This occurs in two contexts:
Immediately following the enrollment SUBSCRIBE the configuration
server MUST send a NOTIFY providing URLs for the configuration
data profiles requested by the UA in the Config-Require header
field of the SUBSCRIBE request. If the configuration server is
not able to provide some specific configuration data profiles or
it does not want the UA to retrieve some specific configuration
profiles at that point in time, it MAY exclude those URL(s) from
the NOTIFY. At a later time when the configuration server is able
to provide the data profile(s) or it wishes the UA to retrieve the
data profiles at that point in time, the configuration server MAY
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send a NOTIFY request containing the URL(s) for the configuration
data profile(s) which the UA SHOULD retrieve immediately.
If the configuration server becomes aware of a configuration
change that it wishes to be effective immediately on the UA, the
configuration server SHOULD send a NOTIFY message containing the
complete list of URLs for the configuration data profiles that the
UA requested when it enrolled. The configuration data profiles
with changed content SHOULD have sequence number larger than that
of the last NOTIFY request. The UA SHOULD retrieve and make
effective the changed configuration URLs immediately upon receipt
of the NOTIFY request. The UA MAY choose to wait to make the
changes effective (e.g. to prevent the change from disrupting
active calls on the UA).
[Do we need an option for the configuration server to tell the UA
that it MUST make the change immediately regardless of state?
Should this be the default?]
The UA SHOULD send a 200 response to the NOTIFY immediately upon
receipt and validation of the solicited request. The configuration
server SHOULD include, in the change notification NOTIFY request,
the complete list of the configuration data profile URLs. The
sequence numbers associated with the configuration data profiles
with changed content should be larger than those in the previous
NOTIFY. These configuration data profile URLs MUST be among those
the UA named in the Config-Requires header field in the most recent
enrollment (SUBSCRIBE request). The URLs listed in the NOTIFY
request MUST use one of the protocols the UA listed in the Config-
Allow header field provided during enrollment in the most recent
SUBSCRIBE request. The sequence numbers for the configuration data
profile URLs are positive integers chosen by the configuration
server. The sequence number value MUST increase monotonically as
modifications are made to a data profile.
This mechanism may be used by the configuration server to provide
firmware updates. For example on a UA that caches or has a
persistent firmware image: if the server realizes (e.g. from the
enrollment information) the UA is running the most currently
available firmware version, it would not provide the URL for the
firmware. However at a later point in time when a new firmware
version was available the configuration server could send a NOTIFY
with the URL for the new firmware version, indicating the UA SHOULD
upgrade now.
4.3.1 NOTIFY Body Content Format
The NOTIFY request contains a body of Content-Type: text/plain. The
content is formatted according to RFC 822 [8]. For each of the
named configuration data profiles which the configuration server is
able to provide, the body contains a header field with the same name
as the configuration data profile. The value of the header field
MUST contain a URL and a sequence number as described in the syntax
below. The protocol of the URL MUST be one of those listed in the
Config-Allow header field provided by the UA in the enrollment
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SUBSCRIBE request. The sequence number associated with the URL is
intended to allow the UA to decide if it has the latest content of
the configuration data profile without having to download and
compare the contents.
Syntax:
config-profile = token : Seq-Param ; Url-Param
Seq-Param = Sequence = 1*digit
Url-Param = Url = tftp-url | Http-url | Https-url
Tftp-url [need reference]
Http-Url as defined in [12, section 3.3]
Httpsw-Url [need reference]
Example:
X-Acme-Special: Sequence=1234567;Url=http://www.acme.com/config.txt
5 Configuration Retrieval
The UA MUST retrieve its configuration data profiles using the URLs
specified by the configuration server in the NOTIFY request. If any
of the retrievals fail, the UA SHOULD re-enroll as described in
section 4. Should the enrollment fail, the UA SHOULD re-discover
the configuration server as described in section 3.
[Is this a good idea? It might cause a nasty cascade effect if the
server for a bunch of URLs goes down. In all likelihood, the
configuration server wont check whether the server is available for
each of the URLs it hands out that would probably be too
expensive. The effect will be to have a bunch of UA spinning back
and forth between hitting the configuration server and hitting the
failed server for the URL. What are the alternatives? Leave the
precise procedure up to the UA since some may need more
sophisticated cutover mechanisms than others. Retry fetching the
URL using an exponential backoff timer between attempts up to some
maximum interval. When that limit is reached, the UA can try to re-
enroll. However, if the configuration server gives it the same
failing URL, it should continue to retry after waiting the maximum
interval timeout.]
6 Configuration Upload
If the UA or another entity wishes to modify a configuration data
profile it MAY make the change persistent on the configuration
server if it is authorized to do so. The configuration server
SHOULD support the ability to upload via the same URL the UA used to
retrieve the configuration data profile. For TFTP the UA does a put
[9]. For HTTP and HTTPS the UA does a POST with a multipart MIME
attachment containing any URL parameters in one part and the changed
configuration data profile [whole or changes only ?? define in
profiles ??] in another part as defined in [?]. If the UA or user
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is not permitted to make the changes on the configuration server the
configuration server returns an HTTP error response code of 403
Forbidden. If the configuration server returns a 403 the UA SHOULD
disallow the changes from being effective on the UA. The UA SHOULD
not make the changes effective until it receives a successful
response (e.g. for HTTP 2xx).
If the URL is for HTTP/HTTPS the server MUST return the changed
configuration data profile in the response (assuming it was
allowed). The configuration server SHOULD include an incremented
sequence number in the HTTP/HTTPS response if the configuration data
profile contents changed [Sip-Ua-Config-Seq header field?]. The UA
SHOULD use the configuration data profile contents from the HTTP
response as opposed to the data that was pushed in the request as
changes may occur from other sources. The configuration server
SHOULD send out a NOTIFY for this change, using the same sequence
number in the configuration data profile URL parameter. This allows
the UA to know that it already has the current contents of the
configuration data profile and SHOULD not download that
configuration data profile.
[TBD in 403 case restrict and provide feedback as to what
specifically is not allowed to be modified by the UA or user]
7 Examples
Below is an example high level message flow for a new UA discovering
and using configuration data from a configuration server. Following
the high level message flows are some specific SIP messages
illustrating SUBSCRIBE and NOTIFY messages from enrollment and
configuration change notification.
7.1 Example Message Flows
The following high level message flows illustrate the configuration
process of discovery, enrollment, configuration retrieval and change
notification with associated configuration retrieval. The UA uses
DHCP with the local option requesting the configuration server
address and port. The DHCP server does not provide the
configuration server address or port. The UA then does a DNS SRV
lookup for the configuration service within the local domain. It
gets a response with one configuration server address and port. The
UA then enrolls with the configuration by sending a SUBSCRIBE
request for the Config-Event. The configuration server sends back a
successful response. The configuration server then sends a NOTIFY
request with the list of URLs for all the configuration data
profiles that the UA named in the enrollment SUBSCRIBE request. The
UA sends a 200 response to the NOTIFY. The UA then downloads all of
the configuration data profiles via the URLs from the NOTIFY
request. The UA is now configured as prescribed.
Later ... an administrator makes a change to the configuration for
the UA on the configuration server. The configuration server on
behalf of the administrator, sends a NOTIFY (change notification)
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request to the UA listing the configuration data profiles that
changed (the minimum subset of the list of configuration data
profiles the UA requested during enrollment). The UA downloads the
configuration data profiles that changed.
UA DHCP Server DNS Server Config. Server
Discovery
IP config. req.
==============>
IP config. wo/ local option
<==============
DNS SRV req. for sipuaconfig service in local domain
=============================>
Host and port for config. server returned
<=============================
Enrollment
SIP SUBSCRIBE Config-Event w/ requested profile names
==================================================>
200 OK
<==================================================
SIP NOTIFY Config-Event w/ requested profile URLs
<==================================================
200 OK
==================================================>
Configuration retrieval
HTTP GET (For each profile URL)
==================================================>
200 OK (specific profile data in body)
<==================================================
.
.
.
Administrative change on configuration server via user interface
.
.
.
Change Notification
SIP NOTIFY Config-Event w/ changed profile URLs
<==================================================
200 OK
==================================================>
HTTP GET (for each changed profile)
==================================================>
200 OK (profile data in body)
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<==================================================
.
.
.
User changes data in a profile on the user agent
.
.
.
Configuration Upload
HTTP POST (changed profile attached as multipart MIME)
==================================================>
200 OK (profile data in body, as change confirmation)
<==================================================
.
.
.
7.2 Example Messages
The following SUBSCRIBE request example is from a UA enrolling with
a configuration server. As this SUBSCRIBE request is for
configuration enrollment the Event header field contains the token
Config-Event. The UA tells the configuration server that it
supports the TFTP, HTTP, HTTPS protocols for retrieving
configuration data profiles in the Config-Allow header field. The
UA tells the configuration server that it would like the
configuration data profiles named: sip-device, sip-user, x-acme-
special in the Config-Require header field. The UA tells the
configuration server that it is enrolling for 86400 seconds via the
Expires header field. During this period of time the configuration
server MUST send a change notification listing the configuration
data profiles which have changed. The UA has identified the
specifics about itself in the From field parameters: Vendor, Model,
Version, Serial, Mac.
UA => Config. Server
SUBSCRIBE sip:config.localdomain.com SIP/2.0
To: sip:config.localdomain.com
From: sip:10.1.1.123;Vendor=acme;Model=model-a
;Version=1.5.0.1;Serial=1234567890;Mac=000aaa1234cd
Call-Id: 987654321@10.1.1.123
Cseq: 1 SUBSCRIBE
Event: Config-Event
Config-Allow: tftp, http, https
Config-Require: Sip-Device, Sip-User, X-Acme-Special, X-Acme-Kernel
Expires: 86400
Content-Length: 0
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The following is an example response to the above enrollment
request.
Config. Server => UA
SIP/2.0 202 Accepted
To: sip:config.localdomain.com
From: sip:10.1.1.123;Vendor=acme;Model=model-a
;Version=1.5.0.1;Serial=1234567890;Mac=000aaa1234cd
Call-Id: 987654321@10.1.1.123
Cseq: 1 SUBSCRIBE
Content-Length: 0
The following example is the immediate NOTITY request the
configuration server sent to the UA following enrollment. The
Config-Expires header field indicates the lease length of the
configuration data profiles. After that period of time if the UA
had not received an additional NOTIFY request from the configuration
server it should re-retrieve the configuration data profiles from
the provided URLs. The URLs are listed in the request body for each
of the named configuration data profiles the UA listed in the
Config-Require header field in the above SUBSCRIBE request from the
UA, that the configuration server is able or wishes to provide.
Note that the configuration server did not provide a URL for X-Acme-
Kernel (perhaps it decided that the kernel image on the UA was
already current).
Config. Server => UA
NOTIFY sip:10.1.1.123 SIP/2.0
To: sip:10.1.1.123;Vendor=acme;Model=model-a
;Version=1.5.0.1;Serial=1234567890;Mac=000aaa1234cd
From: sip:config.localdomain.com
Call-Id: 987654321@10.1.1.123
Cseq: 22 NOTIFY
Event: Config-Event
Config-Expires: 43200
Content-Type: text/plain
Content-Length: 175
Sip-Device: Sequence=1
;Url=http://config.localdomain.com/device/1234567890
Sip-User: Sequence=1;Url=http://config.localdomain.com/user/fred
X-Acme-Special: Sequence=1
;Url=http://www.acme.com/special/config/1234567890
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The following is an example response from the UA for the above
request.
UA => Config. Server
SIP/2.0 200 Ok
To: sip:10.1.1.123;Vendor=acme;Model=model-a
;Version=1.5.0.1;Serial=1234567890;Mac=000aaa1234cd
From: sip:config.localdomain.com
Call-Id: 987654321@10.1.1.123
Cseq: 22 NOTIFY
Content-Length: 0
Assuming at some later point in time, an administrator makes a
change to the content of the Sip-Device configuration data profile
for the UA. The configuration server sends a NOTIFY request to the
UA for the configuration change notification. This example request
below indicates the changed URL(s) in the request body with a higer
sequence number. In this case only one URL has changed, that for
the configuration data profile named: Sip-Device. The configuration
server extends the configuration lease to 43200 seconds from when
this request is received. This lease applies to all of the
configuration data profiles that the UA requested when it last
enrolled. If the UA does not receive another NOTIFY request from
the configuration server before the lease expires, the UA SHOULD
download all of the configuration data profiles from the most recent
URLs provided for each of the configuration data profiles listed in
the enrollment SUBSCRIBE Config-Require header field.
Config. Server => UA
NOTIFY sip:10.1.1.123 SIP/2.0
To: sip:10.1.1.123;Vendor=acme;Model=model-a
;Version=1.5.0.1;Serial=1234567890;Mac=000aaa1234cd
From: sip:config.localdomain.com
Call-Id: 987654321@10.1.1.123
Event: Config-Event
Cseq: 23 NOTIFY
Config-Expires: 43200
Content-Type: text/plain
Content-Length: 64
Sip-Device: Sequence=2
;Url=http://config.localdomain.com/device/1234567890
Sip-User: Sequence=1;Url=http://config.localdomain.com/user/fred
X-Acme-Special: Sequence=1
;Url=http://www.acme.com/special/config/1234567890
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The following is an example response to the above request.
UA => Config. Server
SIP/2.0 200 Ok
To: sip:10.1.1.123;Vendor=acme;Model=model-a
;Version=1.5.0.1;Serial=1234567890;Mac=000aaa1234cd
From: sip:config.localdomain.com
Call-Id: 987654321@10.1.1.123
Cseq: 23 NOTIFY
Content-Length: 0
8 Security Considerations
[This section needs to be greatly expanded and elaborated]
SIP basic and digest authentication [6] MAY be used for
SUBSCRIBE/NOTIFY messages used for enrollment and configuration
change notification. As there is a chicken and egg problem as well
and the content of SUBSCRIBE/NOTIFY messages are transported in the
clear, the credentials that the UA uses in the SUBSCRIBE 401
challenge, or that the configuration server uses in the NOTIFY 401
challenge must be provisioned out of band (i.e. user or
administrator manual input, beamed via PDA, smart card, etc.) via a
secure means.
Configuration data profile URLs are communicated in the clear in the
NOTIFY requests from the configuration server. The security risk of
unauthorized access of the URL content can be mitigated if the
configuration server and UA both support basic authentication and
HTTP or HTTPS. There is a chicken and egg problem here as well
since the content of SUBSCRIBE/NOTIFY messages are transported in
the clear. Accordingly,the credentials that the UA uses for the
HTTP/HTTPS GET/POST 401 challenge must be provisioned out of band
(i.e. user or administrator manual input, beamed via PDA, smart
card, etc.) via a secure means.
Using HTTPS over TLS[13] the configuration server MAY request the
certificate of the UA [14]. If this level of authentication is
desired, the UA vendor SHOULD ship the UA with a digital certificate
or provide a means by which this can be installed out of band. The
configuration server MUST be provisioned with the certificates of
authority allowed for each model of UA to be supported.
Using HTTPS the UA MAY request the certificate of the configuration
server. If this level of authentication is desired the UA must be
provisioned with the allowed certificate(s) of authority and
identities for the configuration server out of band (i.e. user or
administrator manual input, beamed via PDA, smart card, etc.) via a
secure means.
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9 Open Issues
Local DHCP option (i.e. > 128)?
or
DHCP Option for SIP Servers?
How does the configuration server give feedback to a new UA that it
SHOULD/MAY prompt the user for or provide configuration data
specifics?
[Request URI should not contain a user id? The user may not be
known yet.]
[ What happens when the config server receives multiple SUBSCRIBE
requests from the same UA but for different list of profiles. Does
the last request supercede all previous ones?]
[ What happens when the config server receives multiple SUBSCRIBE
requests from the same IP address but for different devices? This
might happen if some entity is acting as a proxy for a bunch of
other devices. It might also happen if the IP address for a UA gets
reused for some other UA (the DHCP lease timeout may be much shorter
than the SUBSCRIBE/NOTIFY lease timeouts). The SUBSCRIBE lease
SHOULD not exceed the DHCP lease? The UA SHOULD re-enroll if its IP
address changes?]
[Do we need an option for the configuration server to tell the UA
that it MUST make the change immediately regardless of state?
Should this be the default?]
[Upload to configuration server configuration data profiles whole or
changes only ?? define in profiles ??]
[Security considerations section needs much elaboration]
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10 References
[1] R. Droms, "Dynamic host configuration protocol," Request for
Comments (Draft Standard) 2131, Internet Engineering Task Force,
Mar. 1997.
[2] S. Alexander and R. Droms, "DHCP options and BOOTP vendor
extensions," Request for Comments (Draft Standard) 2132, Internet
Engineering Task Force, Mar. 1997.
[3] G.Nair, H.Schulzrinne , DHCP Option for SIP Servers,
<draft-ietf-sip-dhcp-01.txt>, IETF; Apr. 2000, Work in progress.
[4] 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.
[5] A. Gulbrandsen, P. Vixie, and L. Esibov, A DNS RR for
specifying the location of services (DNS SRV), Request for
Comments 2782, Internet Engineering Task Force, Feb. 2000.
[6] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg,
SIP: session initiation protocol, Request for Comments 2543,
Internet Engineering Task Force, Mar. 1999.
[7] A. Roach, Event Notification in SIP, <draft-roach-sip-
subscribe-notify-03.txt>, IETF; Feb. 2001, Work in progress.
[8] D. Crocker, STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT
MESSAGES, Request for Comments 822, Internet Engineering Task
Force, Aug. 1982
[9] K. Sollins, THE TFTP PROTOCOL (REVISION 2), Request for
Comments 1350, Internet Engineering Task Force, Jul. 1992
[10] H Schulzrinne, Configuring IP Telephony End Systems,
<schulzrinne-sip-config-00.txt>, IETF; Dec. 2000, Work in
progress
[11] D. Petrie, Requirements for a SIP User Agent Configuration
Framework, <draft-petrie-sip-config-framewk-reqs-00.txt>, IETF;
Feb. 2001, Work in progress
[12] T. Berners-Lee et al, Uniform Resource Locators (URL),
Request for Comments 1738, Internet Engineering Task Force, Dec.
1994
[13] E. Rescorla, HTTP Over TLS, Request for Comments 2818,
Internet Engineering Task Force, May 2000
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User Agent Configuration
[14] T. Dierks, C. Allen, The TLS Protocol Version 1.0, Request
for Comments 2246, Internet Engineering Task Force, Jan. 1999
11 Author's Addresses
Dan Petrie
Pingtel Corp.
400 W. Cummings Park Phone: +1 781 938 5306
Woburn, MA USA Email: dpetrie@pingtel.com
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