Internet Engineering Task Force (IETF) S. Lawrence, Ed.
Request for Comments: 6011 Linden Research, Inc.
Category: Informational J. Elwell
ISSN: 2070-1721 Siemens Enterprise Communications
October 2010
Session Initiation Protocol (SIP) User Agent Configuration
Abstract
This document defines procedures for how a SIP User Agent should
locate, retrieve, and maintain current configuration information from
a Configuration Service.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction ....................................................3
1.1. Scope ......................................................3
1.2. Terminology ................................................3
1.3. User Agent Installation Examples ...........................4
1.3.1. Hosted IP Service Provider Example ..................5
1.3.2. IP-PBX Example ......................................5
1.3.3. Special Considerations for High Security
Deployments .........................................6
2. Obtaining User Agent Configuration ..............................6
2.1. Network Discovery ..........................................6
2.1.1. Link Layer Provisioning .............................7
2.1.2. Network Layer Provisioning ..........................7
2.2. Obtaining the Configuration Service Domain .................8
2.2.1. The Local Network Domain ............................8
2.2.2. Manual Domain Name Entry ............................8
2.3. Constructing the Configuration Request URL .................8
2.3.1. Obtaining a Configuration Service Base URL ..........9
2.3.2. Adding Configuration Request Parameters ............10
2.3.3. Configuration Request URI Example ..................12
2.4. Obtaining Configuration from the Configuration Service ....13
2.4.1. Configuration Data Request Authentication ..........13
2.4.2. Configuration Data Request Failure .................14
2.5. Configuration Changes .....................................15
2.5.1. Configuration Change Subscriptions .................16
2.5.2. Configuration Change Polling .......................18
2.6. Validity of Stored Configuration Data .....................19
2.6.1. Re-Validating Configuration Data ...................19
2.7. Retry Backoff Procedure ...................................20
3. Configuration Data .............................................20
3.1. Configuration Data Items ..................................20
3.1.1. Address-of-Record ..................................21
3.1.2. Realm ..............................................21
3.1.3. Username ...........................................21
3.1.4. Digest .............................................21
3.1.5. OutboundProxy ......................................21
3.2. Reset User Agent to Default Configuration .................21
4. IANA Considerations ............................................21
4.1. DHCP SIP User Agent Configuration Service Domains Option ..21
4.2. DHCPv6 SIP User Agent Configuration Service
Domains Option ............................................22
4.3. U-NAPTR Registration ......................................23
4.4. SIP Forum User Agent Configuration Parameter Registry .....23
5. Security Considerations ........................................24
6. Acknowledgements ...............................................26
7. Normative References ...........................................27
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1. Introduction
A user gets a new SIP User Agent (UA); it may be a hardware device or
software. Some User Agents have a user interface that can accept a
username, password, and domain name. Other devices, like Analog
Telephony Adapters (ATAs), have no user interface other than that
provided by an attached analog phone. How does a non-technical user
minimally configure it so that when it is started, something useful
happens?
1.1. Scope
This document specifies a procedure for how a SIP User Agent locates,
retrieves, and maintains current configuration information for a
given SIP Service Provider. As such, it specifies requirements to be
met by both the User Agent, the Configuration Service at the SIP
Service Provider, and the network infrastructure services that allow
them to communicate.
Nothing in this specification prohibits a User Agent from obtaining
configuration information by any means in addition to the mechanisms
specified herein.
The intent of this specification is to provide mechanisms sufficient
for User Agents to discover an appropriate source of configuration
and maintain the currency of that configuration. A User Agent
implementation compliant with this specification MAY also implement
additional mechanisms necessary in particular environments or when
the services specified here are not available.
The form and content of configuration data to be downloaded are
outside the scope of this specification, although Section 3.1,
"Configuration Data Items" suggests a minimum set of data items
likely to be required by all types of UAs.
1.2. Terminology
The following terms are used in this document:
User Agent, UA
As defined in RFC 3261 [RFC3261]. Note that this includes any
implementation of a User Agent. A SIP phone is a User Agent, but
the term also encompasses any other entity that uses SIP (for
example, for a text chat, for sharing a whiteboard, or for a fax).
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Soft User Agent, Soft UA
A User Agent that runs as an application within some larger system
that has responsibility for some of the steps described in this
specification. In those cases, the Soft UA must be able to obtain
the information from the platform. In all cases, the term User
Agent also encompasses a Soft User Agent.
SIP Service Provider, Service Provider
An entity that provides services to User Agents using the SIP
protocol. This specification requires that a Service Provider
make configuration data and certain other information available in
order to configure User Agents.
Configuration
The set of information that establishes operational parameters for
a particular User Agent.
Configuration Service, CS
The source of Configuration for User Agents.
Configuration Service Domain
The DNS name for the service from which a Configuration is
requested.
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 [RFC2119].
1.3. User Agent Installation Examples
This section is non-normative; it is a set of "user stories" --
narrative descriptions of the user experience in different
environments. These are "black box" descriptions meant to include
the actions to be taken by the human participants (including
administrators and system operators as well as the "user" of the UA),
but not how the network elements communicate or operate internally.
The intent is that these narratives provide context for the
subsequent technical specifications.
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1.3.1. Hosted IP Service Provider Example
Configuring a new UA to use a hosted IP telephony service will
typically proceed as follows: the customer makes a request to their
Service Provider to add one or more new users to their service. The
customer may supply further details such as a preferred username,
type of endpoint and any requests for specific functionality,
depending on what information the Service Provider considers useful,
but no additional information is required from the customer.
The Service Provider performs any necessary provisioning actions on
their equipment, and returns to the customer provisioning
information, which may include a domain name or a numeric domain
identifier for the provider, a user identifier, and a password.
Typically, a Service Provider will supply provisioning information
for each device to be provisioned, but may choose to supply
information that can be used with multiple devices, or for a limited
duration or with other benefits and restrictions.
The customer enters the provisioning information into the UA to be
configured, whereupon the UA uses this information to locate the
configuration service, securely fetch the configuration information,
and configure itself for operation.
1.3.2. IP-PBX Example
Configuring a new UA in a typical business begins by provisioning a
user identity in the Private Branch Exchange (PBX) (add user "John
Smith"), and assigning a phone number to the user. That number must
then be assigned to a line on a specific UA; this is usually done by
selecting a UA and provisioning it in the PBX by its serial number
(usually a Media Access Control (MAC) address), and then assigning
the identity or phone number to a 'line' on that UA in the PBX
configuration system.
Once provisioning in the PBX is complete, the new user goes to his or
her workplace and connects the UA to the network. When connected and
powered up, the UA is provided with the user identity, phone number,
and any other configuration data with no local user interaction --
just connecting it to the network loads the configuration from the
PBX and the UA is operational.
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RFC 6011 SIP UA Configuration October 2010
1.3.3. Special Considerations for High Security Deployments
To deploy a new UA in a high security scenario requires some special
consideration. A security-conscious deployment will most likely
require that the SIP and other management interfaces, including the
interface to the configuration service, be secured before the device
is put into service.
In order to achieve any level of security, the device will need to be
pre-configured with some security-related information in the form of
certificates. This may be achieved in a number of ways. Some
examples include:
1. An administrator who configures the device in a secure
environment before making the device available to the user.
2. Some certificates may be built into the device during the
manufacturing process enabling the configuration service to
certify information such as the manufacturer, UA type, and MAC
address. The configuration service may then be used to provision
the device with other certificates as required.
3. The device may have a facility for the user to provide the
security information in the form of a security card or dongle.
All these mechanism are likely to restrict the user to a limited set
of devices approved for use in a particular deployment.
2. Obtaining User Agent Configuration
This section specifies how a User Agent connects to the network,
determines for which domain to request configuration, obtains
configuration from that domain, and is notified by that domain when
the configuration changes.
The User Agent MAY obtain configuration information by any means in
addition to those specified here, and MAY use such information in
preference to any of the steps specified below, but MUST be capable
of using these procedures alone in order to be compliant with this
specification.
2.1. Network Discovery
A UA needs a minimum set of parameters to allow it to communicate on
the network. Some networks allow the UA to automatically discover
these parameters, while other networks require some or all of these
parameters to be manually provisioned on the UA.
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2.1.1. Link Layer Provisioning
The UA SHOULD attempt to use Link Layer Discovery Protocol - Media
Endpoint Discovery (LLDP-MED; see [ANSI.TIA-1057-2006]) for automatic
provisioning of link layer parameters.
In some deployments, failure to properly provision the link layer may
result in the UA having incorrect Layer 2 priority, degrading the
quality of service, or being on the wrong virtual LAN (VLAN),
possibly resulting in complete loss of service.
2.1.2. Network Layer Provisioning
In order to communicate using IP, the UA needs the following minimal
IP configuration parameters:
IP Network Parameters
o UA IP Address
o Subnet Mask
o Gateway IP address
o DNS Server IP address(es)
With the exception of a Soft UA that relies on its platform to obtain
the IP Network Parameters:
o If the User Agent is using IP version 4 on a network technology
for which the Dynamic Host Configuration Protocol (DHCP) [RFC2131]
is defined, the UA MUST attempt to obtain the IP Network
Parameters using DHCP and MUST request DHCP options 141 (see
Section 4.1) and 15 [RFC2132]. If the DHCP service provides a
value for option 141, the domain name(s) it provides MUST be saved
as candidates for use as the Local Network Domain (see
Section 2.2, "Obtaining the Configuration Service Domain"). If
and only if no values are returned for option 141, the UA MUST
save any values returned for option 15 for use as the Local
Network Domain.
o If the User Agent is using IP version 6 on a network technology
for which the Dynamic Host Configuration Protocol version 6
(DHCPv6) [RFC3315] is defined, the UA MAY use any standard IPv6
mechanism to determine the IP Network Parameters, but MUST request
DHCPv6 options 58 (see Section 4.2) and 21 [RFC3319]. If the
DHCPv6 service provides a value for option 58, those domain names
MUST be saved as candidates for use as the Local Network Domain
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(see Section 2.2, "Obtaining the Configuration Service Domain").
If and only if no values are returned for option 58, the UA MUST
save any values returned for option 21 for use as the Local
Network Domain.
2.2. Obtaining the Configuration Service Domain
To obtain a configuration, the UA needs to know what domain to
request it from. This domain is the Configuration Service Domain;
its value is a DNS name (see [RFC1034]).
User control or prior configuration MAY establish a value for the
Configuration Service Domain that takes precedence over the discovery
procedure defined below. In the absence of user control or prior
configuration, candidate values for the Configuration Service Domain
are obtained as specified in Section 2.2.1, "The Local Network
Domain", or if that is unsuccessful, by the manual mechanism
specified in Section 2.2.2, "Manual Domain Name Entry".
2.2.1. The Local Network Domain
The UA MUST attempt to use each value obtained for the Local Network
Domain name (see Section 2.1.2, "Network Layer Provisioning") as the
Configuration Service Domain. If multiple names are provided by DHCP
and/or DHCPv6 (multiple names may be returned by these services if
both are in use, if the UA has multiple network interfaces, or if the
option responses have multiple values), the UA MUST attempt to use
each of the names provided until a configuration is successfully
obtained. The order in which values obtained in different responses
are used is not defined by this specification -- the UA MAY use any
order; multiple values returned within a single response MUST be
tried in the order they were provided in that response.
If the DHCP service does not provide any local domain name values,
the UA SHOULD use the manual mechanism defined in Section 2.2.2,
"Manual Domain Name Entry".
2.2.2. Manual Domain Name Entry
A UA MAY provide an interface by which a DNS name is supplied
directly by the user for the Configuration Service Name.
2.3. Constructing the Configuration Request URL
Using the Configuration Service Domain name obtained in Section 2.2,
"Obtaining the Configuration Service Domain", the UA MUST construct
an HTTPS URL [RFC2818] with which to request configuration.
Constructing this URL consists of two parts:
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o Section 2.3.1, "Obtaining a Configuration Service Base URL"
o Section 2.3.2, "Adding Configuration Request Parameters"
2.3.1. Obtaining a Configuration Service Base URL
The Configuration Service Domain is resolved to one or more URLs
using the URI-enabled Naming Authority Pointer (U-NAPTR) DDDS
application defined in "Domain-Based Application Service Location
Using URIs and the Dynamic Delegation Discovery Service (DDDS)"
[RFC4848].
The lookup key for the U-NAPTR request is the Configuration Service
Domain name determined in Section 2.2, "Obtaining the Configuration
Service Domain". The UA MUST make a DNS request for NAPTR records
for that domain name. From the returned records, the UA MUST select
those whose Service field value is "SFUA.CFG"; from those records,
the UA MUST extract the HTTPS URL of the Configuration Service from
the Regular Expression field (see next paragraph for the construction
of that field value).
The NAPTR records for the Configuration Service Domain name whose
Service field value is "SFUA.CFG" MUST be configured with the Flag
field set to "U", an empty Substitution field, and a Regular
Expression field value of the following syntax (i.e., a regular
expression to replace the domain name with an https URI):
u-naptr-regexp = "!.*!" <URI> "!"
where <URI> is as defined in STD 66 [RFC3986], the URI syntax
specification, and where the scheme of the URI is "https".
Note that the UA does not need to implement a general regular
expression evaluator in order to process the record above correctly.
The URI value can be extracted by stripping the fixed value "!^.*!"
from the beginning of the value, and "!" from the end of the value to
obtain the base URL. See Section 2.3.3, "Configuration Request URI
Example".
2.3.1.1. Configuration Service Redundancy
Multiple Configuration Servers can be used to provide redundancy and
additional capacity for provisioning User Agents. If the DNS NAPTR
request for the Configuration Service Domain name returns multiple
records with the 'SFUA.CFG' service tag, then the UA should treat the
resulting URLs as alternatives, ordered according to the rules for
the priority and weight as specified for NAPTR records.
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In addition to redundancy provided by multiple NAPTR records,
resolution of the host part of the HTTPS URL can produce multiple
results.
2.3.1.2. Configuration Service Name to Base URL Resolution Failure
If the DNS request to resolve the Configuration Service Domain name
to a request URL does not receive any response, the UA should follow
standard DNS retry procedures.
If the DNS request to resolve the Configuration Service Domain name
to a host name returns a response that indicates that no matching
result is available (NXDOMAIN), the UA SHOULD attempt to obtain
another Configuration Service Domain name using the procedures in
Section 2.2, "Obtaining the Configuration Service Domain".
2.3.2. Adding Configuration Request Parameters
To construct the full configuration request URL, the UA adds one or
more parameters to the base URLs to specify what configuration the UA
is requesting.
1. The UA MUST add all parameters from those defined in the
Configuration Request Parameters list below for which the UA has
a value. Any parameter from that set for which the UA does not
have a value MUST be omitted.
2. The query parameter names defined by this specification all begin
with the prefix 'sfua-'. All names beginning with the prefix
'sfua-' are reserved for this specification and future revisions.
The UA MUST NOT include any request parameter whose name begins
with the prefix 'sfua-' that is not defined by this specification
(including any future revisions).
3. Any parameter not defined by the specification is allowed, but
MUST be ignored by any Configuration Service that does not
recognize it.
2.3.2.1. Configuration Request Parameters
The following parameters are defined for the configuration
request. Section 4.4 creates an IANA registry for these and any
parameters defined in the future.
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sfua-id
The URN identifying the User Agent, constructed as specified in
Section 4.1 of [RFC5626] "Managing Client-Initiated Connections in
the Session Initiation Protocol (SIP)".
Since the procedure defined by [RFC5626] allows any UA to
construct a value for this parameter, the sfua-id parameter MUST
always be included.
If the UA implements [RFC5626], and includes the '+sip.instance'
Contact header field parameter in any request, when requesting
configuration it MUST use the same value for the sfua-id
parameter.
sfua-user
An identifier for a user associated with the configuration. Note
that this might be different than any SIP 'user' in the UA
configuration: it could, for example, be the login name of an
account on the service provider web site. The syntax of this
parameter is that of the 'userid' [RFC2617].
See Section 2.4.1, "Configuration Data Request Authentication" for
how this parameter relates to authentication of the configuration
data request.
sfua-vendor
An identifier that specifies the vendor of the User Agent. The
syntax of the value of this parameter is that of a DNS domain.
The domain value MUST be that of a domain owned by the vendor.
sfua-model
An identifier that further specifies the User Agent from among
those produced by the vendor. The syntax of the value of this
parameter is the same as the 'token' [RFC3261]. Values for this
parameter are selected by the vendor.
sfua-revision
An identifier that further specifies the User Agent from among
those produced by the vendor. The syntax of the value of this
parameter is the same as the 'token' [RFC3261]. Values for this
parameter are selected by the vendor.
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2.3.3. Configuration Request URI Example
Using the rules in Section 2.2, "Obtaining the Configuration Service
Domain", the UA has determined that the Configuration Service Domain
value is "example.net". To obtain the base URL, the UA constructs
the DNS NAPTR request for "example.net.", which returns the DNS
records:
NAPTR 10 10 "u" "SFUA.CFG" "!^.*$!https://p1.example.net/cfg!" ""
NAPTR 100 10 "u" "SFUA.CFG" "!^.*$!https://p2.example.net/cfg!" ""
NAPTR 90 50 "s" "SIP+D2T" "" _sip._tcp.example.net.
NAPTR 100 50 "s" "SIP+D2U" "" _sip._udp.example.net.
Figure 1: Example Configuration Service NAPTR Query Results
The records with the service-field "SFUA.CFG" each provide a base URL
value for SIP UA configuration requests.
Our hypothetical example communications device is a 'HypoComm'
version 2.1, made by ExampleCorp, and has the link layer MAC address
of 00:11:22:33:44:55. It does not have any prior knowledge of a user
identity for which to request configuration, so it constructs query
parameters using the values it does have, combining each with the
base URL to create these request URLs (lines wrapped for
readability):
https://p1.example.net/cfg
?sfua-id=urn:uuid:00000000-0000-1000-8000-001122334455
&sfua-vendor=examplecorp.com
&sfua-model=HypoComm
&sfua-revision=2.1
https://p2.example.net/cfg
?sfua-id=urn:uuid:00000000-0000-1000-8000-001122334455
&sfua-vendor=examplecorp.com
&sfua-model=HypoComm
&sfua-revision=2.1
Figure 2: Example Configuration Request URLs
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2.4. Obtaining Configuration from the Configuration Service
To request configuration using a URL constructed as specified in
Section 2.3, "Constructing the Configuration Request URL" the User
Agent MUST do an HTTPS GET request to each of the URLs until a
configuration that the UA can use is returned in response to one of
the requests.
A successful final response from the Configuration Service to a GET
request for configuration data MUST contain configuration data for
the UA in the HTTP response body. Note that the full capabilities of
HTTP as specified in [RFC2616] are available to the CS, so responses
such as redirection can be used by the CS as a part of the process of
providing configuration data.
Configuration data returned in a successful response is subject to
change by the CS. The HTTP cache control metadata (the max-age
directive value from any Cache-Control header, and the Etag and Last-
Modified header values) returned in the response that provides
configuration data is used to determine when a configuration change
has occurred (Section 2.5.1.3, "Configuration Change Notices") and to
validate any stored configuration data (Section 2.6, "Validity of
Stored Configuration Data").
o An HTTP response from the CS that provides configuration data MUST
include cache control metadata sufficient to ensure that when a
new configuration is available, the cache control information for
that new data is different.
o The UA MUST retain all of the HTTP cache control metadata from any
response that provides configuration data.
2.4.1. Configuration Data Request Authentication
Since the Configuration Request URL scheme is HTTPS, the UA MUST
always use Transport Layer Security (TLS) [RFC5246] to establish a
connection with the Configuration Service.
The UA MUST provide a server_name extension in the TLS Client Hello
message as defined in [RFC4366] "Transport Layer Security (TLS)
Extensions", whose value is the Configuration Service Domain name
(note that this might not be the same as the host part of the CS base
URL). This allows the CS to identify and provide a server
certificate containing the desired identity (allowing for a single
server to serve multiple domain names).
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A UA MUST attempt to validate the server certificate provided by the
CS, in accordance with the rules defined in Section 3.1 of [RFC2818].
Unfortunately, the validation attempt might fail (e.g., because the
UA might not have in firmware a trusted root CA cert to which the CS
certificate chain can be connected or because the root CA cert has
expired since the UA firmware was last updated). If the UA is unable
to validate the server certificate provided by the CS, the UA SHOULD
store the server certificate and alert the user if that CS host
provides a different certificate in the future. Although this 'trust
on first use' model is not as secure as certificate validation, it
does give some protection against man-in-the-middle (MITM) attacks in
the future.
If it has one, the UA MUST provide a client certificate. The CS MUST
validate the UA client's certificate, if one is provided. If the CS
is unable to authenticate the certificate provided by the UA (for
example, the UA is using a self-signed certificate), then the CS MAY
choose to cache the certificate, provided that the UA successfully
authenticates using HTTP authentication (see next paragraph). This
allows a CS to treat the digest authentication credentials as a
single-use password to authenticate the client certificate. This
'trust on first use' model provides protection against future MITM
attacks, provided that the initial communication is not compromised.
If the CS requires HTTP authentication of the configuration data
request, the HTTP 'username' parameter used MUST be the same value as
the sfua-user value provided in the configuration data request
parameters. The UA MUST implement both Basic and Digest
authentication as specified by [RFC2617].
2.4.2. Configuration Data Request Failure
The HTTP configuration data request can fail in a number of ways; the
error handling for each is defined below:
o If a DNS request to resolve the host name in the request URL
returns a response that indicates that no matching result is
available (NXDOMAIN), the UA MUST remove that request URL from the
list of alternatives for the Configuration Service Domain.
o If the attempt to open a TCP connection to the host in the request
URL fails, the UA MAY attempt requests to any alternative URLs for
the same configuration service without waiting between
alternatives, but any requests to the same host MUST wait between
requests according to the procedure defined in Section 2.7, "Retry
Backoff Procedure".
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o If the TCP connection succeeds but the TLS handshake fails,
including failure of the UA to validate the certificate provided
by the Configuration Service host (if the UA is capable of
validation), the UA MUST remove the failed URL from the list of
alternative URLs for this Configuration Service Domain.
o If the request returns a permanent HTTP failure response (response
code >= 400, and does not contain a Retry-After header field), the
UA MUST remove the failed URL from the list of alternatives for
this Configuration Service Domain.
o If the list of alternatives for this Configuration Service Domain
becomes empty, the UA MUST attempt to obtain another Configuration
Service Domain name using the procedures in Section 2.2,
"Obtaining the Configuration Service Domain".
o If the UA has reached its chosen maximum number of retries (this
specification does not specify a maximum number of retries, but
any retries to the same host MUST follow the procedure defined in
Section 2.7, "Retry Backoff Procedure"), the UA MAY attempt to
obtain another Configuration Domain name using the procedures in
Section 2.2, "Obtaining the Configuration Service Domain".
2.5. Configuration Changes
The configuration data provided by the CS is subject to change. This
specification provides for two mechanisms by which the UA discovers
that a configuration change is available:
o SIP subscription by the UA to the CS for notification of changes
to the configuration data.
o HTTP polling by the UA of the configuration data URL at the CS.
The choice of mechanism is made by the Configuration Service and
signaled to the UA in each HTTP response that provides configuration
data. In such a response, the CS MUST either:
o Indicate that the UA is to subscribe for change notifications by
including a Link header in the response with the link relation
'monitor' and SIP URI. This choice is specified in Section 2.5.1,
"Configuration Change Subscriptions".
o Indicate that the UA is to poll for updates using HTTP by not
including a Link header with the link relation 'monitor'. This
choice is specified in Section 2.5.2, "Configuration Change
Polling".
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A User Agent MUST support both mechanisms, and use the mechanism
indicated by the Configuration Service.
2.5.1. Configuration Change Subscriptions
If the CS chooses to use the SIP subscription mechanism, it MUST
include a Link header in the HTTP configuration data response as
specified by [RFC5989]; the URI value in the Link header MUST be a
SIP URI, and the link relation ('rel' attribute) value MUST be
'monitor'. The 'monitor-group' relation MUST NOT be used -- see
below for rules regarding monitoring of multiple configuration data
resources. The SIP URI returned in the Link header is the
'configuration change subscription URI'.
A UA that receives a successful configuration data response with a
Link header that specifies a 'monitor' relation MUST attempt to
maintain a subscription to the SIP URI from the Link header in that
response for the http-monitor event package. This subscription is
referred to herein as a "configuration change subscription".
The CS MUST accept properly authenticated SUBSCRIBE requests from the
UA for the http-monitor event package at the URI it provided in the
Link header of a configuration data response. Authentication of the
SUBSCRIBE request uses any standard SIP authentication mechanism with
credentials supplied to the UA in the configuration data.
Configuration data MAY include references in the form of additional
URLs at the CS that the UA MUST use to obtain additional data. Any
response to requests for these additional URLs that provide
configuration data MUST provide cache control data and a
configuration change subscription URI. The CS MAY return a unique
configuration change subscription URI for each configuration data
request, or MAY return the same SIP URI for different requests, so
long as a change to the configuration data returned in any of these
request results in notification on all subscriptions to the
associated subscription URI.
If the CS returns a unique configuration change subscription URI in
the Link header of different configuration data requests:
o The UA MUST maintain multiple subscriptions; one to each URI
associated with configuration data the UA is using.
If the CS returns the same configuration change subscription URI in
the Link header of different configuration data requests:
o The UA is not required to create multiple subscriptions to the
same URI.
Lawrence & Elwell Informational [Page 16]RFC 6011 SIP UA Configuration October 2010
o The UA MUST associate the URI with each of the configuration data
requests for which it was returned, and any NOTIFY or other change
in the status of that subscription affects the validity of all of
the associated configuration data.
o The CS MUST send a NOTIFY message on the configuration change
subscription when there is a change to any of the different
configuration data resources for which the subscription URI was
returned.
2.5.1.1. Change Subscription Failure
If a configuration change SUBSCRIBE request (either the initial
request or any attempt to refresh the subscription) is permanently
rejected by the Configuration Service (the CS returns a failure
response that is not an authentication challenge or redirection and
does not specify a Retry-After header), the UA MUST consider the
associated configuration data to be not valid and attempt to
revalidate it as specified in Section 2.6.1, "Re-Validating
Configuration Data". Since the CS is not allowed to reject a
properly authenticated request, this indicates a problem either with
the configuration data or the CS.
If a configuration change SUBSCRIBE request (either the initial
request or any attempt to refresh the subscription) fails other than
by being permanently rejected, the UA MUST consider the associated
configuration data to be of unknown validity, and MUST retry the
SUBSCRIBE request as specified in Section 2.7, "Retry Backoff
Procedure"; the maximum time between retries MUST NOT be more than 30
minutes, and the retries MUST continue as long as the configuration
is used. The UA MAY at any time return to any earlier step in the
process of obtaining configuration data.
2.5.1.2. Change Subscription Termination
If the CS explicitly terminates the configuration change (http-
monitor) subscription by sending a NOTIFY message with a
Subscription-State header value of 'terminated', the UA MUST consider
the configuration data to be of unknown validity. If the rules for
interpreting and acting on the 'reason' code parameter as specified
in Section 3.2.4 of [RFC3265] allow, the UA MUST attempt to re-
establish the subscription. If those rules do not allow the UA to
re-subscribe, then the UA MUST consider the data to be not valid and
attempt to revalidate it as specified in Section 2.6.1, "Re-
Validating Configuration Data". The UA MAY at any time return to any
earlier step in the process of obtaining configuration data.
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2.5.1.3. Configuration Change Notices
To inform the UA of a configuration data change, the CS MUST send a
NOTIFY message to the UA in the configuration change subscription
established by the UA as detailed in Section 2.5.1, "Configuration
Change Subscriptions".
The CS MUST NOT send unsolicited (out-of-dialog) NOTIFY messages.
As specified in [RFC5989], the body of a NOTIFY message in the http-
monitor event package is the HTTP headers that would have been
returned in response to an HTTP HEAD request (a HEAD request returns
the headers that would have been returned for a GET request to the
same URI, but with no body).
When a NOTIFY message is received by the UA in the configuration
change subscription, the UA MUST compare the cache control data it
retained when the configuration data was received with the HTTP
header values in the NOTIFY message body. If any of the cache
control data in the HTTP header values differs from those in the
original configuration data response, the UA MUST consider the stored
configuration data to be no longer valid. As soon as reasonably
possible after the UA discovers that configuration data is no longer
valid, the UA MUST attempt a GET request to the HTTPS configuration
request URL which provided the configuration data to obtain the
changed configuration data.
If this HTTPS request to the URL that previously provided the
configuration data fails, the UA MUST attempt to obtain a new URL as
specified in Section 2.3, "Constructing the Configuration Request
URL".
2.5.2. Configuration Change Polling
If the CS chooses to use the HTTP polling mechanism, it MUST NOT
include a Link header with the relation 'monitor' in the HTTP
configuration data response, and MUST include a Cache-Control header
that specifies the max-age directive. The max-age cache control
directive in HTTP specifies the maximum number of seconds for which
the returned data may be cached; this specification defines this time
as being the maximum time the configuration data is considered valid.
A short time before the validity time has passed, the UA SHOULD poll
to revalidate the configuration data as described in Section 2.6.1,
"Re-Validating Configuration Data".
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RFC 6011 SIP UA Configuration October 2010
2.6. Validity of Stored Configuration Data
Configuration data stored by a UA is considered valid:
o If the CS chose to use the subscription mechanism to deliver
change notices, and the UA has a subscription to the CS as
described in Section 2.5.1, "Configuration Change Subscriptions"
on which no NOTIFY message from the CS indicating that the
configuration data has changed has been received.
o If the CS chose to use the HTTP polling method, and the number of
seconds since the configuration data response was received is less
than the time specified by the Cache-Control max-age directive in
that response.
When a UA initializes itself at any time other than immediately after
receiving new configuration data, it MUST consider any stored
configuration data to be of unknown validity.
The UA MAY use configuration data that is of unknown validity, or
configuration data that is known to be no longer valid, while
attempting to revalidate that data or obtain new data. There is no
assurance that such configuration data is still useful, but the UA
MAY choose to retain the data and to continue to use it.
2.6.1. Re-Validating Configuration Data
To revalidate stored configuration data of unknown validity, the UA
MUST repeat the HTTPS GET request it used to obtain the stored
configuration data, with the appropriate HTTP headers to make the
request a conditional request using the cache control data returned
in the response that provided the configuration data. This allows
the CS to respond either with a new configuration data response or a
304 (Not Modified) response to indicate that the configuration data
has not changed.
If the CS responds with a 304 response, and the original response
included a Link header with the 'monitor' relation, the SIP UA MUST
assume that the value of that Link header is also still correct (in
effect, the HTTP cache control values and the subscription URL are a
part of the configuration data), and so the UA MUST attempt to create
and maintain a subscription to that URL as when the configuration
data was first obtained (Section 2.5.1, "Configuration Change
Subscriptions").
Lawrence & Elwell Informational [Page 19]RFC 6011 SIP UA Configuration October 2010
If the CS chooses to use the HTTP polling method, then any 304
response MUST include a Cache-Control header containing a max-age
directive, and the UA MUST use this new value as the maximum validity
time for the associated configuration data.
If the HTTP request to revalidate the configuration fails, the UA
MUST follow the procedures defined for a failure of the initial HTTP
configuration data request as specified in Section 2.4.2,
"Configuration Data Request Failure".
2.7. Retry Backoff Procedure
In case of certain possible failures as described above, the
appropriate response is to retry the failed operation. In all of
these retry cases, the following rules apply:
o The UA SHOULD retry at least 5 times before abandoning the failed
step (except as allowed for in specific error handling rules
above).
o Following the first instance of a given failure, the UA MUST
select an initial backoff timer value randomly between 2 and 8,
inclusive, and wait this number of seconds before retrying the
failed request.
o Following any subsequent instance of a given failure, the UA MUST
increase the backoff timer value by 2 raised to the power of the
number of preceding failures (2^N where N is the number of
previous failures), and wait this increased number of seconds or
the maximum interval specified by specific error handling
procedures, whichever is less, before retrying the failed request.
For example, after an initial failure, the UA randomly chooses an
initial backoff timer value of 4 seconds, followed by retries at the
following times: 6 seconds (4 + 2^1), 10 seconds (6 + 2^2), 18
seconds (10 + 2^3), 34 seconds (18 + 2^4), and 66 seconds (34 + 2^5).
3. Configuration Data
This document does not specify the form or content of configuration
data. As such, the contents of this section are non-normative.
3.1. Configuration Data Items
The configuration data for a SIP UA should, at minimum, include items
with the following semantics.
Lawrence & Elwell Informational [Page 20]RFC 6011 SIP UA Configuration October 2010
3.1.1. Address-of-Record
The Address-of-Record (AOR) is a SIP or SIPS URI that identifies the
user of the device as specified in [RFC3261].
3.1.2. Realm
The realm is used to populate the realm parameter in the SIP Proxy-
Authorization header as specified in [RFC3261] when the UA receives
an authentication challenge.
3.1.3. Username
The username is used to populate the username parameter in the SIP
Proxy-Authorization header as specified in [RFC3261] when the UA
receives an authentication challenge.
3.1.4. Digest
The digest is a string containing the digest of the username, realm,
and password as specified in [RFC2617] and is used to generate a
response to an authentication challenge as specified in [RFC3261].
3.1.5. OutboundProxy
The OutboundProxy if defined contains the default outbound proxy
through which SIP requests, not explicitly routed, are routed as
specified in [RFC3261].
3.2. Reset User Agent to Default Configuration
The earlier sections of this document define methods by which the UA
can be automatically provisioned. Some User Agents allow certain
user specific settings (e.g., Contact Directory, specialized ring-
tones, etc.) to be set by a user, and possibly stored locally in the
User Agent. Since it may be necessary to later re-assign a UA,
designers of configuration data formats may want to provide for
explicit controls for any such locally configured settings, including
the ability to explicitly delete them to return the UA to a
completely unconfigured state.
4. IANA Considerations
4.1. DHCP SIP User Agent Configuration Service Domains Option
This specification defines DHCP option code 141, the "SIP UA
Configuration Service Domains" for inclusion in the IANA registry
"BOOTP Vendor Extensions and DHCP Options" defined by [RFC2939].
Lawrence & Elwell Informational [Page 21]
RFC 6011 SIP UA Configuration October 2010
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 141 | Len | Searchstring... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Searchstring... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the above diagram, Searchstring is a string specifying the
searchlist. If the length of the searchlist exceeds the maximum
permissible within a single option (255 octets), then multiple
options MAY be used, as described in [RFC3396] "Encoding Long DHCP
Options in the Dynamic Host Configuration Protocol (DHCPv4)".
To enable the searchlist to be encoded compactly, searchstrings in
the searchlist MUST be concatenated and encoded using the technique
described in Section 4.1.4 of [RFC1035], "Domain Names -
Implementation and Specification". In this scheme, an entire domain
name or a list of labels at the end of a domain name is replaced with
a pointer to a prior occurrence of the same name. Despite its
complexity, this technique is valuable since the space available for
encoding DHCP options is limited, and it is likely that a domain
searchstring will contain repeated instances of the same domain name.
Thus, the DNS name compression is both useful and likely to be
effective.
For use in this specification, the pointer refers to the offset
within the data portion of the DHCP option (not including the
preceding DHCP option code byte or DHCP option length byte).
If multiple SIP UA Configuration Service Domains options are present,
then the data portions of all the SIP UA Configuration Service
Domains options are concatenated together as specified in RFC 3396,
and the pointer indicates an offset within the complete aggregate
block of data.
For examples of encoding this option, see Section 3 of [RFC3397],
"Dynamic Host Configuration Protocol (DHCP) Domain Search Option",
which uses the same encoding for option 119.
4.2. DHCPv6 SIP User Agent Configuration Service Domains Option
This specification defines DHCPv6 option code 58,
OPTION_SIP_UA_CS_LIST, for inclusion in the IANA registry "Dynamic
Host Configuration Protocol for IPv6 (DHCPv6), DHCP Option Codes"
defined by RFC 3315.
Lawrence & Elwell Informational [Page 22]RFC 6011 SIP UA Configuration October 2010
The format of the SIP User Agent Configuration Service Domains option
is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_SIP_UA_CS_LIST | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| searchlist |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_SIP_UA_CS_LIST (58)
option-len Length of the 'searchlist' field in octets
searchlist The specification of the list of domain names in the SIP
User Agent Configuration Service Domains
The list of domain names in the 'searchlist' MUST be encoded as
specified in Section 8, "Representation and Use of Domain Names" of
RFC 3315.
4.3. U-NAPTR Registration
This document registers the following U-NAPTR application service tag
in the registry defined by [RFC3958], "Domain-Based Application
Service Location Using SRV RRs and the Dynamic Delegation Discovery
Service (DDDS)":
+-------------------------+----------+
| Application Service Tag | SFUA.CFG |
+-------------------------+----------+
This tag is used to obtain the base URL of the Configuration Service
from the DNS name of a SIP domain as specified in Section 2.3.1,
"Obtaining a Configuration Service Base URL".
4.4. SIP Forum User Agent Configuration Parameter Registry
IANA has established a registry for "SIP Forum User Agent
Configuration Parameters". This registry records the HTTPS request
parameters for the initial configuration data request sent by a User
Agent to a Configuration Service as described in Section 2.3.2,
"Adding Configuration Request Parameters".
Lawrence & Elwell Informational [Page 23]RFC 6011 SIP UA Configuration October 2010
Each entry in the registry must include the Parameter Name and a
Description that specifies the value syntax and usage of the
parameter:
Parameter Name The name of the parameter, which MUST match the ABNF
production for 'token' from [RFC3261].
Value Syntax The syntax of the value, if any (a parameter may just
be a name with no associated value).
Usage The purpose served by the parameter, including any
default method the UA should use to construct it if
applicable.
The initial values for the registry are the parameters described in
Section 2.3.2.1, "Configuration Request Parameters". The policy for
future additions to this registry depends on the parameter name
value:
If the name of the parameter begins with the characters 'sfua-' in
any case, then the policy for addition to this registry is "RFC
Required" as described by [RFC5226].
Any other parameter entry may be added to this registry using a
"First Come First Served" policy as described by [RFC5226].
5. Security Considerations
Initial discovery of the Configuration Service Domain name relies on
a number of operations that are normally unsecured: a DHCP response
could be provided by an attacker to replace the values of any of the
IP Network Parameters (Section 2.1.2, "Network Layer Provisioning")
including the Local Network Domain which is the default choice for
the Configuration Service Domain name. Confirmation by the human
user of the Configuration Service Domain name, especially when it
differs from a previously used value, could be used to mitigate this
(perhaps unintentional) potential reconfiguration. Note that
previously loaded configuration MAY constrain which parts of the
discovery and location procedures are used: for example, the
Configuration Service Domain name might be fixed so that it cannot be
modified by discovery.
The connection to the Configuration Service is made over TLS. As the
TLS server, the CS always provides a server certificate during the
TLS handshake; if possible, the UA should validate that certificate
and confirm that it contains as a subject the Configuration Service
Domain name or at least the host name from the Configuration Service
Base URL (see [RFC2818]). While it may not be possible to have the
Lawrence & Elwell Informational [Page 24]RFC 6011 SIP UA Configuration October 2010
information needed to perform a full validation of the CS server
certificate prior to the first configuration (for example, the UA may
not have a current CA certificate for the CA that signs the CS server
certificate), implementors are advised to provide that information in
configuration data so that it can be used for subsequent
reconfigurations; this narrows the window of vulnerability to the
first configuration attempt.
To secure initial configuration attempts, the CS can deny requests
from unknown devices and/or could implement other measures such as
restricting the time window during which it will accept an initial
configuration request from a given device. A more secure approach
would be to provide the user with a password, perhaps a one-time
password valid only for the initial access. In high security
environments, the Configuration Service could require that the User
Agent provide a client certificate for authentication in the TLS
connection for configuration data requests. This would necessitate
some prior manual configuration of the User Agent, and possibly the
Configuration Service, and that configuration should also include
sufficient information for the UA to fully validate the CS
certificate.
The values of some or all of the request parameters sent by the UA on
the initial request for configuration data (see Section 2.3.2,
"Adding Configuration Request Parameters") may be sensitive
information. Since the configuration data request is made over a TLS
connection, the confidentiality of that information is protected on
the network. Configuration Service implementations should take all
necessary measures to ensure that the request parameter data is
appropriately protected within the CS itself.
The Configuration Change Request Subscription (Section 2.5.1,
"Configuration Change Subscriptions") is established only after the
configuration data has been loaded by the User Agent, so all security
mechanisms available in SIP (including request digest authentication
and the use of TLS) can be configured and required by either the CS
or the UA. Note that a configuration change notice does not actually
provide any new configuration data, nor can it change where the UA
sends a request for the new configuration data. This means that an
attacker cannot reconfigure a UA by subverting only the change notice
subscription; the most the attacker can do is trigger checks for new
data. In order to actually modify the configuration data itself, the
attacker must subvert the CS or the steps leading to the CS discovery
(subject to the checks described above).
Implementations of TLS typically support multiple versions of the
Transport Layer Security protocol as well as the older Secure Sockets
Layer (SSL) protocol. Because of known security vulnerabilities, SIP
Lawrence & Elwell Informational [Page 25]
RFC 6011 SIP UA Configuration October 2010
UAs, SIP Service Provider, and the Configuration Service Host MUST
NOT request, offer, or use SSL 2.0. See Appendix E.2 of [RFC5246]
for further details.
6. Acknowledgements
Contributing Members of the SIP Forum User Agent Configuration
Working Group:
Francois Audet, Nortel Networks, Inc.
Eric Burger, SIP Forum
Sumanth Channabasappa, Cable Television Laboratories, Inc.
(CableLabs)
Martin Dolly, AT&T Labs
John Elwell, Siemens Enterprise Communications
Marek Dutkiewicz, Polycom, Inc.
Andy Hutton, Siemens Enterprise Communications
Lincoln Lavoie, University of New Hampshire
Scott Lawrence, Avaya, Inc.
Paul Mossman, Avaya, Inc.
Michael Procter, VoIP.co.uk
Marc Robins, SIP Forum
Henning Schulzrinne, Columbia University
Rifaat Shekh-Yusef, Avaya, Inc.
Robert Sparks, Tekelec
Simo Veikkolainen, Nokia
The Editor would like to also acknowledge valuable contributions by
Leslie Daigle and Margaret Wasserman.
Lawrence & Elwell Informational [Page 26]RFC 6011 SIP UA Configuration October 2010
7. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC2616] 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.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC2939] Droms, R., "Procedures and IANA Guidelines for Definition
of New DHCP Options and Message Types", BCP 43, RFC 2939,
September 2000.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3319] Schulzrinne, H. and B. Volz, "Dynamic Host Configuration
Protocol (DHCPv6) Options for Session Initiation Protocol
(SIP) Servers", RFC 3319, July 2003.
Lawrence & Elwell Informational [Page 27]RFC 6011 SIP UA Configuration October 2010
[RFC3396] Lemon, T. and S. Cheshire, "Encoding Long Options in the
Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
November 2002.
[RFC3397] Aboba, B. and S. Cheshire, "Dynamic Host Configuration
Protocol (DHCP) Domain Search Option", RFC 3397,
November 2002.
[RFC3958] Daigle, L. and A. Newton, "Domain-Based Application
Service Location Using SRV RRs and the Dynamic Delegation
Discovery Service (DDDS)", RFC 3958, January 2005.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
[RFC4848] Daigle, L., "Domain-Based Application Service Location
Using URIs and the Dynamic Delegation Discovery Service
(DDDS)", RFC 4848, April 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5626] Jennings, C., Mahy, R., and F. Audet, "Managing Client-
Initiated Connections in the Session Initiation Protocol
(SIP)", RFC 5626, October 2009.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5989] Roach, A., "A SIP Event Package for Subscribing to Changes
to an HTTP Resource", October 2010.
[ANSI.TIA-1057-2006]
American National Standards Institute, "Telecommunications
IP Telephony Infrastructure Link Layer Discovery Protocol
for Media Endpoint Devices", April 1993.
Lawrence & Elwell Informational [Page 28]RFC 6011 SIP UA Configuration October 2010
Authors' Addresses
Scott Lawrence (editor)
Linden Research, Inc.
EMail: scott-ietf@skrb.org
John Elwell
Siemens Enterprise Communications
Phone: +44 1908 817801
EMail: john.elwell@siemens-enterprise.com
Lawrence & Elwell Informational [Page 29]
