SIP Telephony Device Requirements and Configuration

Status of This Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2006).

Abstract

This document describes the requirements for SIP telephony devices, based on the deployment experience of large numbers of SIP phones and PC clients using different implementations in various networks. The objectives of the requirements are a well-defined set of interoperability and multi-vendor-supported core features, so as to enable similar ease of purchase, installation, and operation as found for PCs, PDAs, analog feature phones or mobile phones.

We present a glossary of the most common settings and some of the more widely used values for some settings.

Table of Contents

   1. Introduction ....................................................3
      1.1. Conventions used in this document ..........................4
   2. Generic Requirements ............................................4
      2.1. SIP Telephony Devices ......................................4
      2.2. DNS and ENUM Support .......................................5
      2.3. SIP Device Resident Telephony Features .....................5
      2.4. Support for SIP Services ...................................8
      2.5. Basic Telephony and Presence Information Support ...........9
      2.6. Emergency and Resource Priority Support ....................9
      2.7. Multi-Line Requirements ...................................10
      2.8. User Mobility .............................................11
      2.9. Interactive Text Support ..................................11
   
      2.10. Other Related Protocols ..................................12
      2.11. SIP Device Security Requirements .........................13
      2.12. Quality of Service .......................................13
      2.13. Media Requirements .......................................14
      2.14. Voice Codecs .............................................14
      2.15. Telephony Sound Requirements .............................15
      2.16. International Requirements ...............................15
      2.17. Support for Related Applications .........................16
      2.18. Web-Based Feature Management .............................16
      2.19. Firewall and NAT Traversal ...............................16
      2.20. Device Interfaces ........................................17
   3. Glossary and Usage for the Configuration Settings ..............18
      3.1. Device ID .................................................18
      3.2. Signaling Port ............................................19
      3.3. RTP Port Range ............................................19
      3.4. Quality of Service ........................................19
      3.5. Default Call Handling .....................................19
           3.5.1. Outbound Proxy .....................................19
           3.5.2. Default Outbound Proxy .............................20
           3.5.3. SIP Session Timer ..................................20
      3.6. Telephone Dialing Functions ...............................20
           3.6.1. Phone Number Representations .......................20
           3.6.2. Digit Maps and/or the Dial/OK Key ..................20
           3.6.3. Default Digit Map ..................................21
      3.7. SIP Timer Settings ........................................21
      3.8. Audio Codecs ..............................................21
      3.9. DTMF Method ...............................................22
      3.10. Local and Regional Parameters ............................22
      3.11. Time Server ..............................................22
      3.12. Language .................................................23
      3.13. Inbound Authentication ...................................23
      3.14. Voice Message Settings ...................................23
      3.15. Phonebook and Call History ...............................24
      3.16. User-Related Settings and Mobility .......................24
      3.17. AOR-Related Settings .....................................25
      3.18. Maximum Connections ......................................25
      3.19. Automatic Configuration and Upgrade ......................25
      3.20. Security Configurations ..................................26
   4. Security Considerations ........................................26
      4.1. Threats and Problem Statement .............................26
      4.2. SIP Telephony Device Security .............................27
      4.3. Privacy ...................................................28
      4.4. Support for NAT and Firewall Traversal ....................28
   5. Acknowledgements ...............................................29
   6. Informative References .........................................31

1. Introduction

This document has the objective of focusing the Internet communications community on requirements for telephony devices using SIP.

We base this information from developing and using a large number of SIP telephony devices in carrier and private IP networks and on the Internet. This deployment has shown the need for generic requirements for SIP telephony devices and also the need for some specifics that can be used in SIP interoperability testing.

SIP telephony devices, also referred to as SIP User Agents (UAs), can be any type of IP networked computing user device enabled for SIP- based IP telephony. SIP telephony user devices can be SIP phones, adaptors for analog phones and for fax machines, conference speakerphones, software packages (soft clients) running on PCs, laptops, wireless connected PDAs, 'Wi-Fi' SIP mobile phones, as well as other mobile and cordless phones that support SIP signaling for real-time communications. SIP-PSTN gateways are not the object of this memo, since they are network elements and not end user devices.

SIP telephony devices can also be instant messaging (IM) applications that have a telephony option.

SIP devices MAY support various other media besides voice, such as text, video, games, and other Internet applications; however, the non-voice requirements are not specified in this document, except when providing enhanced telephony features.

SIP telephony devices are highly complex IP endpoints that speak many Internet protocols, have audio and visual interfaces, and require functionality targeted at several constituencies: (1) end users, (2) service providers and network administrators, (3) manufacturers, and (4) system integrators.

The objectives of the requirements are a well-defined set of interoperability and multi-vendor-supported core features, so as to enable similar ease of purchase, installation, and operation as found for standard PCs, analog feature phones, or mobile phones. Given the cost of some feature-rich display phones may approach the cost of PCs and PDAs, similar or even better ease of use as compared to personal computers and networked PDAs is expected by both end users and network administrators.

While some of the recommendations of this document go beyond what is currently mandated for SIP implementations within the IETF, this is believed necessary to support the specified operational objectives.

However, it is also important to keep in mind that the SIP specifications are constantly evolving; thus, these recommendations need to be considered in the context of that change and evolution. Due to the evolution of IETF documents in the standards process, and the informational nature of this memo, the references are all informative.

1.1. Conventions used in this document

This document is informational and therefore the key words "MUST", "SHOULD", "SHOULD NOT", and "MAY", in this document are not to be interpreted as described in RFC 2119 [1], but rather indicate the nature of the suggested requirement.

2. Generic Requirements

We present here a minimal set of requirements that MUST be met by all SIP [2] telephony devices, except where SHOULD or MAY is specified.

2.1. SIP Telephony Devices

This memo applies mainly to desktop phones and other special purpose SIP telephony hardware. Some of the requirements in this section are not applicable to PC/laptop or PDA software phones (soft phones) and mobile phones.

Req-1:

SIP telephony devices MUST be able to acquire IP network

settings by automatic configuration using Dynamic Host

Configuration Protocol (DHCP) [3].

Req-2:

SIP telephony devices MUST be able to acquire IP network

settings by manual entry of settings from the device.

Req-3:

SIP telephony devices SHOULD support IPv6. Some newer

wireless networks may mandate support for IPv6 and in such

networks SIP telephony devices MUST support IPv6.

Req-4:

SIP telephony devices MUST support the Simple Network Time Protocol [4].

Req-5:

Desktop SIP phones and other special purpose SIP telephony devices MUST be able to upgrade their firmware to support additional features and the functionality.

Req-6:

Users SHOULD be able to upgrade the devices with no special applications or equipment; or a service provider SHOULD be able to push the upgrade down to the devices remotely.

2.2. DNS and ENUM Support

Req-7:

SIP telephony devices MUST support RFC 3263 [5] for locating a SIP server and selecting a transport protocol.

Req-8:

SIP telephony devices MUST incorporate DNS resolvers that are configurable with at least two entries for DNS servers for redundancy. To provide efficient DNS resolution, SIP telephony devices SHOULD query responsive DNS servers and skip DNS servers that have been non-responsive to recent queries.

Req-9:

To provide efficient DNS resolution and to limit post-dial delay, SIP telephony devices MUST cache DNS responses based on the DNS time-to-live.

Req-10:

For DNS efficiency, SIP telephony devices SHOULD use the additional information section of the DNS response instead of generating additional DNS queries.

Req-11:

SIP telephony devices MAY support ENUM [6] in case the end users prefer to have control over the ENUM lookup. Note: The ENUM resolver can also be placed in the outgoing SIP proxy to simplify the operation of the SIP telephony device. The Extension Mechanisms for DNS (EDNSO) in RFC 2671 SHOULD also be supported.

2.3. SIP Device Resident Telephony Features

Req-12:

SIP telephony devices MUST support RFC 3261 [2].

Req-13:

SIP telephony devices SHOULD support the SIP Privacy header by populating headers with values that reflect the privacy requirements and preferences as described in "User Agent Behavior", Section 4 of RFC 3323 [7].

Req-14:

SIP telephony devices MUST be able to place an existing call on hold, and initiate or receive another call, as specified in RFC 3264 [8] and SHOULD NOT omit the sendrecv attribute.

Req-15:

SIP telephony devices MUST provide a call waiting indicator. When participating in a call, the user MUST be alerted audibly and/or visually of another incoming call. The user MUST be able to enable/disable the call waiting indicator.

Req-16:

SIP telephony devices MUST support SIP message waiting [9] and the integration with message store platforms.

Req-17:

SIP telephony devices MAY support a local dial plan. If a dial plan is supported, it MUST be able to match the user input to one of multiple pattern strings and transform the input to a URI, including an arbitrary scheme and URI parameters.

Example: If a local dial plan is supported, it SHOULD be configurable to generate any of the following URIs when "5551234" is dialed:

   tel:+12125551234
   sip:+12125551234@example.net;user=phone
   sips:+12125551234@example.net;user=phone
   sip:5551234@example.net
   sips:5551234@example.net
   tel:5551234;phone-context=nyc1.example.net
   sip:5551234;phone-
   context=nyc1.example.net@example.net;user=phone
   sips:5551234;phone-
   context=nyc1.example.net@example.net;user=phone
   sip:5551234;phone-
   context=nyc1.example.net@example.net;user=dialstring
   sips:5551234;phone-
   context=nyc1.example.net@example.net;user=dialstring
   tel:5551234;phone-context=+1212
   sip:5551234;phone-context=+1212@example.net;user=phone
   sips:5551234;phone-context=+1212@example.net;user=phone
   sip:5551234;phone-context=+1212@example.net;user=dialstring
   sips:5551234;phone-context=+1212@example.net;user=dialstring

If a local dial plan is not supported, the device SHOULD be configurable to generate any of the following URIs when "5551234" is dialed:

   sip:5551234@example.net
   sips:5551234@example.net
   sip:5551234;phone-
   context=nyc1.example.net@example.net;user=dialstring
   sips:5551234;phone-
   context=nyc1.example.net@example.net;user=dialstring
   sip:5551234;phone-context=+1212@example.net;user=dialstring
   sips:5551234;phone-context=+1212@example.net;user=dialstring"

Req-18:

SIP telephony devices MUST support URIs for telephone numbers as per RFC 3966 [10]. This includes the reception as well as the sending of requests. The reception may be denied according to the configurable security policy of the device. It is a reasonable behavior to send a request to a preconfigured outbound proxy.

Req-19:

SIP telephony devices MUST support REFER and NOTIFY for call transfer [11], [12]. SIP telephony devices MUST support escaped Replaces-Header (RFC 3891) and SHOULD support other escaped headers in the Refer-To header.

Req-20:

SIP telephony devices MUST support the unattended call transfer flows as defined in [12].

Req-21:

SIP telephony devices MUST support the attended call transfer as defined in [12].

   Req-22: SIP telephony devices MAY support device-based 3-way calling
           by mixing the audio streams and displaying the interactive
           text of at least 2 separate calls.

Req-23:

SIP telephony devices MUST be able to send dual-tone multi- frequency (DTMF) named telephone events as specified by RFC 2833 [13].

Req-24:

Payload type negotiation MUST comply with RFC 3264 [8] and with the registered MIME types for RTP payload formats in RFC 3555 [14].

Req-25:

The dynamic payload type MUST remain constant throughout the session. For example, if an endpoint decides to renegotiate codecs or put the call on hold, the payload type for the re- invite MUST be the same as the initial payload type. SIP devices MAY support Flow Identification as defined in RFC 3388 [15].

Req-26:

When acting as a User Agent Client (UAC), SIP telephony devices SHOULD support the gateway model of RFC 3960 [16]. When acting as a User Agent Server (UAS), SIP telephony devices SHOULD NOT send early media.

Req-27:

SIP telephony devices MUST be able to handle multiple early dialogs in the context of request forking. When a confirmed dialog has been established, it is an acceptable behavior to send a BYE request in response to additional 2xx responses that establish additional confirmed dialogs.

Req-28:

SIP devices with a suitable display SHOULD support the call- info header and depending on the display capabilities MAY, for example, display an icon or the image of the caller.

Req-29:

To provide additional information about call failures, SIP telephony devices with a suitable display MUST render the "Reason Phrase" of the SIP message or map the "Status Code" to custom or default messages. This presumes the language for the reason phrase is the same as the negotiated language. The devices MAY use an internal "Status Code" table if there was a problem with the language negotiation.

Req-30:

SIP telephony devices MAY support music on hold, both in receive mode and locally generated. See also "SIP Service Examples" for a call flow with music on hold [17].

Req-31:

SIP telephony devices MAY ring after a call has been on hold for a predetermined period of time, typically 3 minutes.

2.4. Support for SIP Services

Req-32:

SIP telephony devices MUST support the SIP Basic Call Flow Examples as per RFC 3665 [17].

Req-33:

SIP telephony devices MUST support the SIP-PSTN Service Examples as per RFC 3666 [18].

Req-34:

SIP telephony devices MUST support the Third Party Call Control model [19], in the sense that they may be the controlled device.

Req-35:

SIP telephony devices SHOULD support SIP call control and multi-party usage [20].

Req-36:

SIP telephony devices SHOULD support conferencing services for voice [21], [22] and interactive text [23] and if equipped with an adequate display MAY also support instant messaging (IM) and presence [24], [25].

Req-37:

SIP telephony devices SHOULD support the indication of the

User Agent capabilities and MUST support the caller

capabilities and preferences as per RFC 3840 [26].

Req-38:

SIP telephony devices MAY support service mobility: Devices MAY allow roaming users to input their identity so as to have access to their services and preferences from the home SIP server. Examples of user data to be available for roaming users are: user service ID, dialing plan, personal directory, and caller preferences.

2.5. Basic Telephony and Presence Information Support

The large color displays in some newer models make such SIP phones and applications attractive for a rich communication environment. This document is focused, however, only on telephony-specific features enabled by SIP Presence and SIP Events.

SIP telephony devices can also support presence status, such as the traditional Do Not Disturb, new event state-based information, such as being in another call or being in a conference, typing a message, emoticons, etc. Some SIP telephony User Agents can support, for example, a voice session and several IM sessions with different parties.

Req-39:

SIP telephony devices SHOULD support Presence information [24] and SHOULD support the Rich Presence Information Data Format [27] for the new IP communication services enabled by Presence.

Req-40:

Users MUST be able to set the state of the SIP telephony device to "Do Not Disturb", and this MAY be manifested as a Presence state across the network if the UA can support Presence information.

Req-41:

SIP telephony devices with "Do Not Disturb" enabled MUST respond to new sessions with "486 Busy Here".

2.6. Emergency and Resource Priority Support

   Req-42: Emergency calling: For emergency numbers (e.g., 911, SOS
           URL), SIP telephony devices SHOULD support the work of the
           ECRIT WG [28].

Req-43:

Priority header: SIP devices SHOULD support the setting by the user of the Priority header specified in RFC 3261 for such applications as emergency calls or for selective call acceptance.

Req-44:

Resource Priority header: SIP telephony devices that are used in environments that support emergency preparedness MUST also support the sending and receiving of the Resource-Priority header as specified in [29]. The Resource Priority header influences the behavior for message routing in SIP proxies and PSTN telephony gateways and is different from the SIP Priority header specified in RFC 3261. Users of SIP telephony devices may want to be interrupted in their lower- priority communications activities if such an emergency communication request arrives.

Note: As of this writing, we recommend that implementers follow the work of the Working Group on Emergency Context Resolution with Internet Technologies (ecrit) in the IETF. The complete solution is for further study at this time. There is also work on the requirements for location conveyance in the SIPPING WG, see [30].

2.7. Multi-Line Requirements

A SIP telephony device can have multiple lines: One SIP telephony device can be registered simultaneously with different SIP registrars from different service providers, using different names and credentials for each line. The different sets of names and credentials are also called 'SIP accounts'. The "line" terminology has been borrowed from multi-line PSTN/PBX phones, except that for SIP telephony devices there can be different SIP registrars/proxies for each line, each of which may belong to a different service provider, whereas this would be an exceptional case for the PSTN and certainly not the case for PBX phones. Multi-line SIP telephony devices resemble more closely e-mail clients that can support several e-mail accounts.

Note: Each SIP account can usually support different Addresses of Record (AORs) with a different list of contact addresses (CAs), as may be convenient, for example, when having different SIP accounts for business and personal use. However, some of the CAs in different SIP accounts may point to the same devices.

Req-45:

Multi-line SIP telephony devices MUST support a unique authentication username, authentication password, registrar, and identity to be provisioned for each line. The authentication username MAY be identical with the user name of the AOR and the domain name MAY be identical with the host name of the registrar.

Req-46:

Multi-line SIP telephony devices MUST be able to support the state of the client to Do Not Disturb on a per line basis.

   Req-47: Multi-line SIP telephony devices MUST support multi-line call
           waiting indicators.  Devices MUST allow the call waiting
           indicator to be set on a per line basis.

Req-48:

Multi-line SIP telephony devices MUST be able to support a few different ring tones for different lines. We specify here "a few", since provisioning different tones for all lines may be difficult for phones with many lines.

2.8. User Mobility

The following requirements allow users with a set of credentials to use any SIP telephony device that can support personal credentials from several users, distinct from the identity of the device.

   Req-49: User-mobility-enabled SIP telephony devices MUST store static
           credentials associated with the device in non-volatile
           memory.  This static profile is used during the power up
           sequence.
   
   Req-50: User-mobility-enabled SIP telephony devices SHOULD allow a
           user to walk up to a device and input their personal
           credentials.  All user features and settings stored in home
           SIP proxy and the associated policy server SHOULD be
           available to the user.
   
   Req-51: User-mobility-enabled SIP telephony devices registered as
           fixed desktop with network administrator MUST use the local
           static location data associated with the device for emergency
           calls.

2.9. Interactive Text Support

SIP telephony devices supporting instant messaging based on SIMPLE [24] support text conversation based on blocks of text. However, continuous interactive text conversation may be sometimes preferred as a parallel to voice, due to its interactive and more streaming- like nature, and thus is more appropriate for real-time conversation. It also allows for text captioning of voice in noisy environments and for those who cannot hear well or cannot hear at all.

Finally, continuous character-by-character text is preferred by emergency and public safety programs (e.g., 112 and 911) because of its immediacy, efficiency, lack of crossed messages problem, better ability to interact with a confused person, and the additional information that can be observed from watching the message as it is composed.

Req-52:

SIP telephony devices such as SIP display phones and IP- analog adapters SHOULD support the accessibility requirements for deaf, hard-of-hearing and speech-impaired individuals as per RFC 3351 [31] and also for interactive text conversation [23], [32].

Req-53:

SIP telephony devices SHOULD provide a way to input text and to display text through any reasonable method. Built-in user interfaces, standard wired or wireless interfaces, and/or support for text through a web interface are all considered reasonable mechanisms.

Req-54:

SIP telephony devices SHOULD provide an external standard wired or wireless link to connect external input (keyboard, mouse) and display devices.

Req-55:

SIP telephony devices that include a display, or have a facility for connecting an external display, MUST include protocol support as described in RFC 4103 [23] for real-time interactive text.

Req-56:

There may be value in having RFC 4103 support in a terminal also without a visual display. A synthetic voice output for the text conversation may be of value for all who can hear, and thereby provides the opportunity to have a text conversation with other users.

Req-57:

SIP telephony devices MAY provide analog adaptor functionality through an RJ-11 FXS port to support FXS devices. If an RJ-11 (FXS) port is provided, then it MAY support a gateway function from all text-telephone protocols according to ITU-T Recommendation V.18 to RFC 4103 text conversation (in fact, this is encouraged in the near term during the transition to widespread use of SIP telephony devices). If this gateway function is not included or fails, the device MUST pass through all text-telephone protocols according to ITU-T Recommendation V.18, November 2000, in a transparent fashion.

Req-58:

SIP telephony devices MAY provide a 2.5 mm audio port, in portable SIP devices, such as PDAs and various wireless SIP phones.

2.10. Other Related Protocols

Req-59:

SIP telephony devices MUST support the Real-Time Protocol and the Real-Time Control Protocol, RFC 3550 [33]. SIP devices SHOULD use RTCP Extended Reports for logging and reporting on network support for voice quality, RFC 3611 [34] and MAY also support the RTCP summary report delivery [35].

2.11. SIP Device Security Requirements

Req-60:

SIP telephony devices MUST support digest authentication as per RFC 3261. In addition, SIP telephony devices MUST support Transport Layer Security (TLS) for secure transport [36] for scenarios where the SIP registrar is located outside the secure, private IP network in which the SIP UA may reside. Note: TLS need not be used in every call, though.

Req-61:

SIP telephony devices MUST be able to password protect configuration information and administrative functions.

Req-62:

SIP telephony devices MUST NOT display the password to the user or administrator after it has been entered.

Req-63:

SIP clients MUST be able to disable remote access, i.e., block incoming Simple Network Management Protocol (SNMP) (where this is supported), HTTP, and other services not necessary for basic operation.

Req-64:

SIP telephony devices MUST support the option to reject an incoming INVITE where the user-portion of the SIP request URI is blank or does not match a provisioned contact. This provides protection against war-dialer attacks, unwanted telemarketing, and spam. The setting to reject MUST be configurable.

Req-65:

When TLS is not used, SIP telephony devices MUST be able to reject an incoming INVITE when the message does not come from the proxy or proxies where the client is registered. This prevents callers from bypassing terminating call features on the proxy. For DNS SRV specified proxy addresses, the client must accept an INVITE from all of the resolved proxy IP addresses.

2.12. Quality of Service

Req-66:

SIP devices MUST support the IPv4 Differentiated Services Code Point (DSCP) field for RTP streams as per RFC 2597 [37]. The DSCP setting MUST be configurable to conform with the local network policy.

Req-67:

If not specifically provisioned, SIP telephony devices SHOULD mark RTP packets with the recommended DSCP for expedited forwarding (codepoint 101110) and mark SIP packets with DSCP AF31 (codepoint 011010).

Req-68:

SIP telephony devices MAY support Resource Reservation Protocol (RSVP) [38].

2.13. Media Requirements

Req-69:

To simplify the interoperability issues, SIP telephony devices MUST use the first matching codec listed by the receiver if the requested codec is available in the called device. See the offer/answer model in RFC 3261.

Req-70:

To reduce overall bandwidth, SIP telephony devices MAY support active voice detection and comfort noise generation.

2.14. Voice Codecs

Internet telephony devices face the problem of supporting multiple codecs due to various historic reasons, on how telecom industry players have approached codec implementations and the serious intellectual property and licensing problems associated with most codec types. For example, RFC 3551 [39] lists 17 registered MIME subtypes for audio codecs.

Ideally, the more codecs can be supported in a SIP telephony device, the better, since it enhances the chances of success during the codec negotiation at call setup and avoids media intermediaries used for codec mediation.

Implementers interested in a short list MAY, however, support a minimal number of codecs used in wireline Voice over IP (VoIP), and also codecs found in mobile networks for which the SIP UA is targeted. An ordered short list of preferences may look as follows:

Req-71:

SIP telephony devices SHOULD support Audio/Video Transport (AVT) payload type 0 (G.711 uLaw) as in [40] and its Annexes 1 and 2.

Req-72:

SIP telephony devices SHOULD support the Internet Low Bit Rate codec (iLBC) [41], [42].

Req-73:

Mobile SIP telephony devices MAY support codecs found in various wireless mobile networks. This can avoid codec conversion in network-based intermediaries.

Req-74:

SIP telephony devices MAY support a small set of special purpose codecs, such as G.723.1, where low bandwidth usage is needed (for dial-up Internet access), Speex [43], or G.722 for high-quality audio conferences.

Req-75:

SIP telephony devices MAY support G.729 and its annexes. Note: The G.729 codec is included here for backward compatibility only, since the iLBC and the G.723.1 codecs are preferable in bandwidth-constrained environments.

Note: The authors believe the Internet Low Bit Rate codec (iLBC) should be the default codec for Internet telephony.

A summary count reveals up to 25 and more voice codec types currently in use. The authors believe there is also a need for a single multi-rate Internet codec, such as Speex or similar that can effectively be substituted for all of the multiple legacy G.7xx codec types, such as G.711, G.729, G.723.1, G.722, etc., for various data rates, thus avoiding the complexity and cost to implementers and service providers alike who are burdened by supporting so many codec types, besides the licensing costs.

2.15. Telephony Sound Requirements

Req-76:

SIP telephony devices SHOULD comply with the handset receive comfort noise requirements outlined in the ANSI standards [44], [45].

Req-77:

SIP telephony devices SHOULD comply with the stability or minimum loss defined in ITU-T G.177.

Req-78:

SIP telephony devices MAY support a full-duplex speakerphone function with echo and side tone cancellation. The design of high-quality side tone cancellation for desktop IP phones, laptop computers, and PDAs is outside the scope of this memo.

Req-79:

SIP telephony device MAY support different ring tones based on the caller identity.

2.16. International Requirements

Req-80:

SIP telephony devices SHOULD indicate the preferred language [46] using User Agent capabilities [26].

Req-81:

SIP telephony devices intended to be used in various language settings MUST support other languages for menus, help, and labels.

2.17. Support for Related Applications

The following requirements apply to functions placed in the SIP telephony device.

Req-82:

SIP telephony devices that have a large display and support presence SHOULD display a buddy list [24].

Req-83:

SIP telephony devices MAY support Lightweight Directory Access Protocol (LDAP) for client-based directory lookup.

Req-84:

SIP telephony devices MAY support a phone setup where a URL is automatically dialed when the phone goes off-hook.

2.18. Web-Based Feature Management

Req-85:

SIP telephony devices SHOULD support an internal web server to allow users the option to manually configure the phone and to set up personal phone applications such as the address book, speed-dial, ring tones, and, last but not least, the call handling options for the various lines and aliases, in a user-friendly fashion. Web pages to manage the SIP telephony device SHOULD be supported by the individual device, or MAY be supported in managed networks from centralized web servers linked from a URI.

Managing SIP telephony devices SHOULD NOT require special client software on the PC or require a dedicated management console. SIP telephony devices SHOULD support https transport for this purpose.

In addition to the Web Based Feature Management requirement, the device MAY have an SNMP interface for monitoring and management purposes.

2.19. Firewall and NAT Traversal

The following requirements allow SIP clients to properly function behind various firewall architectures.

Req-86:

SIP telephony devices SHOULD be able to operate behind a static Network Address Translation/Port Address Translation (NAPT) device. This implies the SIP telephony device SHOULD be able to 1) populate SIP messages with the public, external address of the NAPT device; 2) use symmetric UDP or TCP for signaling; and 3) use symmetric RTP [47].

Req-87:

SIP telephony devices SHOULD support the Simple Traversal of UDP through NATs (STUN) protocol [48] for determining the NAPT public external address. A classification of scenarios and NATs where STUN is effective is reported in [49]. Detailed call flows for interactive connectivity establishment (ICE) [50] are given in [51].

Note: Developers are strongly advised to follow the document on best current practices for NAT traversal for SIP [51].

   Req-88: SIP telephony devices MAY support UPnP (http://www.upnp.org/)
           for local NAPT traversal.  Note that UPnP does not help if
           there is NAPT in the network of the service provider.

Req-89:

SIP telephony devices MUST be able to limit the ports used for RTP to a provisioned range.

2.20. Device Interfaces

Req-90:

SIP telephony devices MUST support two types of addressing capabilities, to enable end users to "dial" either phone numbers or URIs.

Req-91:

SIP telephony devices MUST have a telephony-like dial-pad and MAY have telephony-style buttons such as mute, redial, transfer, conference, hold, etc. The traditional telephony dial-pad interface MAY appear as an option in large-screen telephony devices using other interface models, such as Push-To-Talk in mobile phones and the Presence and IM graphical user interface (GUI) found in PCs, PDAs, mobile phones, and cordless phones.

Req-92:

SIP telephony devices MUST have a convenient way for entering SIP URIs and phone numbers. This includes all alphanumeric characters allowed in legal SIP URIs. Possible approaches include using a web page, display and keyboard entry, type- ahead, or graffiti for PDAs.

Req-93:

SIP telephony devices should allow phone number entry in human-friendly fashion, with the usual separators and brackets between digits and digit groups.

3. Glossary and Usage for the Configuration Settings

SIP telephony devices are quite complex, and their configuration is made more difficult by the widely diverse use of technical terms for the settings. We present here a glossary of the most common settings and some of the more widely used values for some settings.

Settings are the information on a SIP UA that it needs so as to be a functional SIP endpoint. The settings defined in this document are not intended to be a complete listing of all possible settings. It MUST be possible to add vendor-specific settings.

The list of available settings includes settings that MUST, SHOULD, or MAY be used by all devices (when present) and that make up the common denominator that is used and understood by all devices. However, the list is open to vendor-specific extensions that support additional settings, which enable a rich and valuable set of features.

Settings MAY be read-only on the device. This avoids the misconfiguration of important settings by inexperienced users generating service cost for operators. The settings provisioning process SHOULD indicate which settings can be changed by the end user and which settings should be protected.

In order to achieve wide adoption of any settings format, it is important that it should not be excessive in size for modest devices to use it. Any format SHOULD be structured enough to allow flexible extensions to it by vendors. Settings may belong to the device or to a SIP service provider and the Address of Record (AOR) registered there. When the device acts in the context of an AOR, it will first try to look up a setting in the AOR context. If the setting cannot be found in that context, the device will try to find the setting in the device context. If that also fails, the device MAY use a default value for the setting.

The examples shown here are just of informational nature. Other documents may specify the syntax and semantics for the respective settings.

3.1. Device ID

A device setting MAY include some unique identifier for the device it represents. This MAY be an arbitrary device name chosen by the user, the MAC address, some manufacturer serial number, or some other unique piece of data. The Device ID SHOULD also indicate the ID type.
Example: DeviceId="000413100A10;type=MAC"

3.2. Signaling Port

The port that will be used for a specific transport protocol for SIP MAY be indicated with the SIP ports setting. If this setting is omitted, the device MAY choose any port within a range as specified in 3.3. For UDP, the port may also be used for sending requests so that NAT devices will be able to route the responses back to the UA. Example: SIPPort="5060;transport=UDP"

3.3. RTP Port Range

A range of port numbers MUST be used by a device for the consecutive pairs of ports that MUST be used to receive audio and control information (RTP and RTCP) for each concurrent connection. Sometimes this is required to support firewall traversal, and it helps network operators to identify voice packets. Example: RTPPorts="50000-51000"

3.4. Quality of Service

The Quality of Service (QoS) settings for outbound packets SHOULD be configurable for network packets associated with call signaling (SIP) and media transport (RTP/RTCP). These settings help network operators in identifying voice packets in their network and allow them to transport them with the required QoS. The settings are independently configurable for the different transport layers and signaling, media, or administration. The QoS settings SHOULD also include the QoS mechanism.

For both categories of network traffic, the device SHOULD permit configuration of the type of service settings for both layer 3 (IP DiffServ) and layer 2 (for example, IEEE 802.1D/Q) of the network protocol stack. Example: RTPQoS="0xA0;type=DiffSrv,5;type=802.1DQ;vlan=324"

3.5. Default Call Handling

All of the call handling settings defined below can be defined here as default behaviors.

3.5.1. Outbound Proxy

The outbound proxy for a device MAY be set. The setting MAY require that all signaling packets MUST be sent to the outbound proxy or that only in the case when no route has been received the outbound proxy MUST be used. This ensures that application layer gateways are in the signaling path. The second requirement allows the optimization of the routing by the outbound proxy. Example: OutboundProxy="sip:nat.proxy.com"

3.5.2. Default Outbound Proxy

The default outbound proxy SHOULD be a global setting (not related to a specific line).
Example: DefaultProxy="sip:123@proxy.com"

3.5.3. SIP Session Timer

The re-invite timer allows User Agents to detect broken sessions caused by network failures. A value indicating the number of seconds for the next re-invite SHOULD be used if provided. Example: SessionTimer="600;unit=seconds"

3.6. Telephone Dialing Functions

As most telephone users are used to dialing digits to indicate the address of the destination, there is a need for specifying the rule by which digits are transformed into a URI (usually SIP URI or TEL URI).

3.6.1. Phone Number Representations

SIP phones need to understand entries in the phone book of the most common separators used between dialed digits, such as spaces, angle and round brackets, dashes, and dots.
Example: A phonebook entry of "+49(30)398.33-401" should be translated into "+493039833401".

3.6.2. Digit Maps and/or the Dial/OK Key

A SIP UA needs to translate user input before it can generate a valid request. Digit maps are settings that describe the parameters of this process. If present, digit maps define patterns that when matched define the following:

   1) A rule by which the endpoint can judge that the user has completed
      dialing, and
   2) A rule to construct a URI from the dialed digits, and optionally
   3) An outbound proxy to be used in routing the SIP INVITE.

A critical timer MAY be provided that determines how long the device SHOULD wait before dialing if a dial plan contains a T (Timer) character. It MAY also provide a timer for the maximum elapsed time that SHOULD pass before dialing if the digits entered by the user match no dial plan. If the UA has a Dial or OK key, pressing this key will override the timer setting.

SIP telephony devices SHOULD have a Dial/OK key. After sending a request, the UA SHOULD be prepared to receive a 484 Address Incomplete response. In this case, the UA should accept more user input and try again to dial the number.

An example digit map could use regular expressions like in DNS NAPTR (RFC 2915) to translate user input into a SIP URL. Additional replacement patterns like "d" could insert the domain name of the used AOR. Additional parameters could be inserted in the flags portion of the substitution expression. A list of those patterns would make up the dial plan:

   |^([0-9]*)#$|sip:\1@\d;user=phone|outbound=proxy.com
   |^([a-zA-Z0-9&=+\$,;?\-_.!~*'()%]+@.+)|sip:\1|
   |^([a-zA-Z0-9&=+\$,;?\-_.!~*'()%]+)$|sip:\1@\d|
   |^(.*)$|sip:\1@\d|timeout=5

3.6.3. Default Digit Map

The SIP telephony device SHOULD support the configuration of a default digit map. If the SIP telephony device does not support digit maps, it SHOULD at least support a default digit map rule to construct a URI from digits. If the endpoint does support digit maps, this rule applies if none of the digit maps match.

For example, when a user enters "12345", the UA might send the request to "sip:12345@proxy.com;user=phone" after the user presses the OK key.

3.7. SIP Timer Settings

The parameters for SIP (like timer T1) and other related settings MAY be indicated. An example of usage would be the reduction of the DNS SRV failover time.
Example: SIPTimer="t1=100;unit=ms"

Note:

The timer settings can be included in the digit map.

3.8. Audio Codecs

In some cases, operators want to control which codecs may be used in their network. The desired subset of codecs supported by the device SHOULD be configurable along with the order of preference. Service providers SHOULD have the possibility of plugging in their own codecs of choice. The codec settings MAY include the packet length and other parameters like silence suppression or comfort noise generation.

The set of available codecs will be used in the codec negotiation according to RFC 3264. Example: Codecs="speex/8000;ptime=20;cng=on,gsm;ptime=30"

The settings MUST include hints about privacy for audio using Secure Realtime Transport Protocol (SRTP) that either mandate or encourage the usage of secure RTP.
Example: SRTP="mandatory"

3.9. DTMF Method

Keyboard interaction can be indicated with in-band tones or preferably with out-of-band RTP packets (RFC 2833 [13]). The method for sending these events SHOULD be configurable with the order of precedence. Settings MAY include additional parameters like the content-type that should be used. Example: DTMFMethod="INFO;type=application/dtmf, RFC2833".

3.10. Local and Regional Parameters

Certain settings are dependent upon the regional location for the daylight saving time rules and for the time zone.

Time Zone and UTC Offset: A time zone MAY be specified for the user. Where one is specified; it SHOULD use the schema used by the Olson Time One database [52].

Examples of the database naming scheme are Asia/Dubai or America/Los Angeles where the first part of the name is the continent or ocean and the second part is normally the largest city in that time zone. Optional parameters like the UTC offset may provide additional information for UAs that are not able to map the time zone information to a internal database. Example: TimeZone="Asia/Dubai;offset=7200"

3.11. Time Server

A time server SHOULD be used. DHCP is the preferred way to provide this setting. Optional parameters may indicate the protocol that SHOULD be used for determining the time. If present, the DHCP time server setting has higher precedence than the time server setting. Example: TimeServer="12.34.5.2;protocol=NTP"

3.12. Language

Setting the correct language is important for simple installation around the globe.

A language setting SHOULD be specified for the whole device. Where it is specified, it MUST use the codes defined in RFC 3066 to provide some predictability.
Example: Language="de"

It is recommended to set the language as writable, so that the user MAY change this. This setting SHOULD NOT be AOR related.

A SIP UA MUST be able to parse and accept requests containing international characters encoded as UTF-8 even if it cannot display those characters in the user interface.

3.13. Inbound Authentication

SIP allows a device to limit incoming signaling to those made by a predefined set of authorized users from a list and/or with valid passwords. Note that the inbound proxy from most service providers may also support the screening of incoming calls, but in some cases users may want to have control in the SIP telephony device for the screening.

A device SHOULD support the setting as to whether authentication (on the device) is required and what type of authentication is required. Example: InboundAuthentication="digest;pattern=*"

If inbound authentication is enabled, then a list of allowed users and credentials to call this device MAY be used by the device. The credentials MAY contain the same data as the credentials for an AOR (i.e., URL, user, password digest, and domain). This applies to SIP control signaling as well as call initiation.

3.14. Voice Message Settings

Various voice message settings require the use of URIs for the service context as specified in RFC 3087 [53].

The message waiting indicator (MWI) address setting controls where the client SHOULD SUBSCRIBE to a voice message server and what MWI summaries MAY be displayed [9]. Example: MWISubscribe="sip:mailbox01@media.proxy.com"

User Agents SHOULD accept MWI information carried by SIP MESSAGE without prior subscription. This way the setup of voice message settings can be avoided.

3.15. Phonebook and Call History

The UA SHOULD have a phonebook and keep a history of recent calls. The phonebook SHOULD save the information in permanent memory that keeps the information even after restarting the device or save the information in an external database that permanently stores the information.

3.16. User-Related Settings and Mobility

A device MAY specify the user that is currently registered on the device. This SHOULD be an address-of-record URL specified in an AOR definition.

The purpose of specifying which user is currently assigned to this device is to provide the device with the identity of the user whose settings are defined in the user section. This is primarily interesting with regards to user roaming. Devices MAY allow users to sign on to them and then request that their particular settings be retrieved. Likewise, a user MAY stop using a device and want to disable their AOR while not present. For the device to understand what to do, it MUST have some way of identifying users and knowing which user is currently using it. By separating the user and device properties, it becomes clear what the user wishes to enable or to disable. Providing an identifier in the configuration for the user gives an explicit handle for the user. For this to work, the device MUST have some way of identifying users and knowing which user is currently assigned to it.

One possible scenario for roaming is an agent who has definitions for several AORs (e.g., one or more personal AORs and one for each executive for whom the administrator takes calls) that they are registered for. If the agent goes to the copy room, they would sign on to a device in that room and their user settings including their AOR would roam with them.

The alternative to this is to require the agent to individually configure each of the AORs (this would be particularly irksome using standard telephone button entry).

The management of user profiles, aggregation of user or device AOR, and profile information from multiple management sources are configuration server concerns that are out of the scope of this document. However, the ability to uniquely identify the device and user within the configuration data enables easier server-based as well as local (i.e., on the device) configuration management of the configuration data.

3.17. AOR-Related Settings

SIP telephony devices MUST use the AOR-related settings, as specified here.

There are many properties which MAY be associated with or SHOULD be applied to the AOR or signaling addressed to or from the AOR. AORs MAY be defined for a device or a user of the device. At least one AOR MUST be defined in the settings; this MAY pertain to either the device itself or the user.
Example: AOR="sip:12345@proxy.com"

It MUST be possible to specify at least one set of domain, user name, and authentication credentials for each AOR. The user name and authentication credentials are used for authentication challenges.

3.18. Maximum Connections

A setting defining the maximum number of simultaneous connections that a device can support MUST be used by the device. The endpoint might have some maximum limit, most likely determined by the media handling capability. The number of simultaneous connections may be also limited by the access bandwidth, such as of DSL, cable, and wireless users. Other optional settings MAY include the enabling or disabling of call waiting indication.

A SIP telephony device MAY support at least two connections for three-way conference calls that are locally hosted. Example: MaximumConnections="2;cwi=false;bw=128".

See the recent work on connection reuse [54] and the guidelines for connection-oriented transport for SIP [55].

3.19. Automatic Configuration and Upgrade

Automatic SIP telephony device configuration SHOULD use the processes and requirements described in [56]. The user name or the realm in the domain name SHOULD be used by the configuration server to automatically configure the device for individual- or group-specific settings, without any configuration by the user. Image and service data upgrades SHOULD also not require any settings by the user.

3.20. Security Configurations

The device configuration usually contains sensitive information that MUST be protected. Examples include authentication information, private address books, and call history entries. Because of this, it is RECOMMENDED to use an encrypted transport mechanism for configuration data. Where devices use HTTP, this could be TLS.

For devices which use FTP or TFTP for content delivery this can be achieved using symmetric key encryption.

Access to retrieving configuration information is also an important issue. A configuration server SHOULD challenge a subscriber before sending configuration information.

The configuration server SHOULD NOT include passwords through the automatic configuration process. Users SHOULD enter the passwords locally.

4. Security Considerations

4.1. Threats and Problem Statement

While Section 2.11 states the minimal security requirements and NAT/firewall traversal that have to be met respectively by SIP telephony devices, developers and network managers have to be aware of the larger context of security for IP telephony, especially for those scenarios where security may reside in other parts of SIP- enabled networks.

Users of SIP telephony devices are exposed to many threats [57] that include but are not limited to fake identity of callers, telemarketing, spam in IM, hijacking of calls, eavesdropping, and learning of private information such as the personal phone directory, user accounts and passwords, and the personal calling history. Various denial of service (DoS) attacks are possible, such as hanging up on other people's conversations or contributing to DoS attacks of others.

Service providers are also exposed to many types of attacks that include but are not limited to theft of service by users with fake identities, DoS attacks, and the liabilities due to theft of private customer data and eavesdropping in which poorly secured SIP telephony devices or especially intermediaries such as stateful back-to-back user agents with media (B2BUA) may be implicated.

SIP security is a hard problem for several reasons:

      o Peers can communicate across domains without any pre-arranged
        trust relationship.
      o There may be many intermediaries in the signaling path.
      o Multiple endpoints can be involved in such telephony operations
        as forwarding, forking, transfer, or conferencing.
      o There are seemingly conflicting service requirements when
        supporting anonymity, legal intercept, call trace, and privacy.
      o Complications arise from the need to traverse NATs and
        firewalls.

There are a large number of deployment scenarios in enterprise networks, using residential networks and employees using Virtual Private Network (VPN) access to the corporate network when working from home or while traveling. There are different security scenarios for each. The security expectations are also very different, say, within an enterprise network or when using a laptop in a public wireless hotspot, and it is beyond the scope of this memo to describe all possible scenarios in detail.

The authors believe that adequate security for SIP telephony devices can be best implemented within protected networks, be they private IP networks or service provider SIP-enabled networks where a large part of the security threats listed here are dealt with in the protected network. A more general security discussion that includes network- based security features, such as network-based assertion of identity [58] and privacy services [7], is outside the scope of this memo, but must be well understood by developers, network managers, and service providers.

In the following, some basic security considerations as specified in RFC 3261 are discussed as they apply to SIP telephony devices.

4.2. SIP Telephony Device Security

Transport Level Security

SIP telephony devices that operate outside the perimeter of secure private IP networks (this includes telecommuters and roaming users) MUST use TLS to the outgoing SIP proxy for protection on the first hop. SIP telephony devices that use TLS must support SIPS in the SIP headers.

Supporting large numbers of TLS channels to endpoints is quite a burden for service providers and may therefore constitute a premium service feature.

Digest Authentication

SIP telephony devices MUST support digest authentication to register with the outgoing SIP registrar. This ensures proper identity credentials that can be conveyed by the network to the called party. It is assumed that the service provider operating the outgoing SIP registrar has an adequate trust relationship with its users and knows its customers well enough (identity, address, billing relationship, etc.). The exceptions are users of prepaid service. SIP telephony devices that accept prepaid calls MUST place "unknown" in the "From" header.

End User Certificates

SIP telephony devices MAY store personal end user certificates that are part of some Public Key Infrastructure (PKI) [59] service for high-security identification to the outgoing SIP registrar as well as for end-to-end authentication. SIP telephony devices equipped for certificate-based authentication MUST also store a key ring of certificates from public certificate authorities (CAs).

Note the recent work in the IETF on certificate services that do not require the telephony devices to store certificates [60].

End-to-End Security Using S/MIME

S/MIME [61] MUST be supported by SIP telephony devices to sign and encrypt portions of the SIP message that are not strictly required for routing by intermediaries. S/MIME protects private information in the SIP bodies and in some SIP headers from intermediaries. The end user certificates required for S/MIME ensure the identity of the parties to each other. Note: S/MIME need not be used, though, in every call.

4.3. Privacy

Media Encryption

Secure RTP (SRTP) [62] MAY be used for the encryption of media such as audio, text, and video, after the keying information has been passed by SIP signaling. Instant messaging MAY be protected end-to-end using S/MIME.

4.4. Support for NAT and Firewall Traversal

The various NAT and firewall traversal scenarios require support in telephony SIP devices. The best current practices for NAT traversal for SIP are reviewed in [51]. Most scenarios where there are no SIP-enabled network edge NAT/firewalls or gateways in the enterprise can be managed if there is a STUN client in the SIP telephony device and a STUN server on the Internet, maintained by a service provider. In some exceptional cases (legacy symmetric NAT), an external media relay must also be provided that can support the Traversal Using Relay NAT (TURN) protocol exchange with SIP telephony devices. Media relays such as TURN come at a high bandwidth cost to the service provider, since the bandwidth for many active SIP telephony devices must be supported. Media relays may also introduce longer paths with additional delays for voice.

Due to these disadvantages of media relays, it is preferable to avoid symmetric and non-deterministic NATs in the network, so that only STUN can be used, where required. Reference [63] deals in more detail how NAT has to 'behave'.

It is not always obvious to determine the specific NAT and firewall scenario under which a SIP telephony device may operate.

For this reason, the support for Interactive Connectivity Establishment (ICE) has been defined to be deployed in all devices that required end-to-end connectivity for SIP signaling and RTP media streams, as well as for streaming media using Real Time Streaming Protocol (RTSP). ICE makes use of existing protocols, such as STUN and TURN.

Call flows using SIP security mechanisms

The high-level security aspects described here are best illustrated by inspecting the detailed call flows using SIP security, such as in [64].

Security enhancements, certificates, and identity management

As of this writing, recent work in the IETF deals with the SIP Authenticated Identity Body (AIB) format [65], new S/MIME requirements, enhancements for the authenticated identity, and Certificate Management Services for SIP. We recommend developers and network managers to follow this work as it will develop into IETF standards.

5. Acknowledgements

Paul Kyzivat and Francois Audet have made useful comments how to support to the dial plan requirements in Req-17. Mary Barnes has kindly made a very detailed review of version 04 that has contributed to significantly improving the document. Useful comments on version 05 have also been made by Ted Hardie, David Kessens, Russ Housley, and Harald Alvestrand that are reflected in this version of the document.

We would like to thank Jon Peterson for very detailed comments on the previous version 0.3 that has prompted the rewriting of much of this document. John Elwell has contributed with many detailed comments on version 04 of the document. Rohan Mahy has contributed several clarifications to the document and leadership in the discussions on support for the hearing disabled. These discussions have been concluded during the BOF on SIP Devices held during the 57th IETF, and the conclusions are reflected in the section on interactive text support for hearing- or speech-disabled users.

Gunnar Hellstrom, Arnoud van Wijk, and Guido Gybels have been instrumental in driving the specification for support of the hearing disabled.

The authors would also like to thank numerous persons for contributions and comments to this work: Henning Schulzrinne, Jorgen Bjorkner, Jay Batson, Eric Tremblay, David Oran, Denise Caballero McCann, Brian Rosen, Jean Brierre, Kai Miao, Adrian Lewis, and Franz Edler. Jonathan Knight has contributed significantly to earlier versions of the requirements for SIP phones. Peter Baker has also provided valuable pointers to TIA/EIA IS 811 requirements to IP phones that are referenced here.

Last but not least, the co-authors of the previous versions, Daniel Petrie and Ian Butcher, have provided support and guidance all along in the development of these requirements. Their contributions are now the focus of separate documents.

6. Informative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
   
        Levels", BCP 14, RFC 2119, March 1997.
   
   [2]  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.
   
   [3]  Lemon, T. and S. Cheshire, "Encoding Long Options in the Dynamic
        Host Configuration Protocol (DHCPv4)", RFC 3396, November 2002.
   
   [4]  Mills, D., "Simple Network Time Protocol (SNTP) Version 4 for
        IPv4, IPv6 and OSI", RFC 4330, January 2006.
   
   [5]  Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
        (SIP): Locating SIP Servers", RFC 3263, June 2002.
   
   [6]  Peterson, J., "enumservice registration for Session Initiation
        Protocol (SIP) Addresses-of-Record", RFC 3764, April 2004.
   
   [7]  Peterson, J., "A Privacy Mechanism for the Session Initiation
        Protocol (SIP)", RFC 3323, November 2002.
   
   [8]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
        Session Description Protocol (SDP)", RFC 3264, June 2002.
   
   [9]  Mahy, R., "A Message Summary and Message Waiting Indication
        Event Package for the Session Initiation Protocol (SIP)", RFC
        3842, August 2004.
   
   [10] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966,
        December 2004.
   
   [11] Sparks, R., "The Session Initiation Protocol (SIP) Refer
        Method", RFC 3515, April 2003.
   
   [12] Johnston, A., "SIP Service Examples", Work in Progress, March
        2006.
   
   [13] Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF Digits,
        Telephony Tones and Telephony Signals", RFC 2833, May 2000.
   
   [14] Casner, S. and P. Hoschka, "MIME Type Registration of RTP
        Payload Formats", RFC 3555, July 2003.
   
   [15] Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne,
        "Grouping of Media Lines in the Session Description Protocol
        (SDP)", RFC 3388, December 2002.
   
   [16] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone
        Generation in the Session Initiation Protocol (SIP)", RFC 3960,
        December 2004.

[17] Johnston, A., Donovan, S., Sparks, R., Cunningham, C., and K.

Summers, "Session Initiation Protocol (SIP) Basic Call Flow Examples", BCP 75, RFC 3665, December 2003.

[18] Johnston, A., Donovan, S., Sparks, R., Cunningham, C., and K.

Summers, "Session Initiation Protocol (SIP) Public Switched Telephone Network (PSTN) Call Flows", BCP 76, RFC 3666, December 2003.

   [19] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
        "Best Current Practices for Third Party Call Control (3pcc) in
        the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
        2004.
   
   [20] Mahy, R., et al., "A Call Control and Multi-party usage
        framework for the Session Initiation Protocol (SIP)", Work in
        Progress, March 2006.
   
   [21] Johnston, A. and O. Levin, "Session Initiation Protocol Call
        Control - Conferencing for User Agents", Work in Progress,
        October 2005.
   
   [22] Even, R. and N. Ismail, "Conferencing Scenarios", Work in
        Progress, September 2005.
   
   [23] Hellstrom, G. and P. Jones, "RTP Payload for Text Conversation",
        RFC 4103, June 2005.
   
   [24] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and
        D. Gurle, "Session Initiation Protocol (SIP) Extension for
        Instant Messaging", RFC 3428, December 2002.
   
   [25] Rosenberg, J., "A Presence Event Package for the Session
        Initiation Protocol (SIP)", RFC 3856, August 2004.
   
   [26] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating User
        Agent Capabilities in the Session Initiation Protocol (SIP)",
        RFC 3840, August 2004.
   
   [27] Schulzrinne, H., Gurbani, V., Kyzivat, P., and J. Rosenberg,
        "RPID: Rich Presence Extensions to the Presence Information Data
        Format (PIDF)", Work in Progress, September 2005.

[28] See the Working Group on Emergency Context Resolution with

        Internet Technologies at
        http://www.ietf.org/html.charters/ecrit-charter.html
   
   [29] Schulzrinne, H. and J. Polk, "Communications Resource Priority
        for the Session Initiation Protocol (SIP)", RFC 4412, February
        2006.
   
   [30] Polk, J. and B. Rosen, "Session Initiation Protocol Location
        Conveyance", Work in Progress, July 2005.
   
   [31] Charlton, N., Gasson, M., Gybels, G., Spanner, M., and A. van
        Wijk, "User Requirements for the Session Initiation Protocol
        (SIP) in Support of Deaf, Hard of Hearing and Speech-impaired
        Individuals", RFC 3351, August 2002.
   
   [32] van Wijk, A., "Framework of requirements for real-time text
        conversation using SIP", Work in Progress, September 2005.
   
   [33] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
        "RTP: A Transport Protocol for Real-Time Applications", STD 64,
        RFC 3550, July 2003.
   
   [34] Friedman, T., Caceres, R., and A. Clark, "RTP Control Protocol
        Extended Reports (RTCP XR)", RFC 3611, November 2003.
   
   [35] Pendleton, A., "SIP Package for Quality Reporting Event", Work
        in Progress, February 2006.
   
   [36] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
        2246, January 1999.
   
   [37] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski, "Assured
        Forwarding PHB Group", RFC 2597, June 1999.
   
   [38] Braden, R., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
        "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
        Specification", RFC 2205, September 1997.
   
   [39] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
        Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
   
   [40] ITU-T Recommendation G.711, available online at
        http://www.itu.int.
   
   [41] Andersen, S., Duric, A., Astrom, H., Hagen, R., Kleijn, W., and
        J. Linden, "Internet Low Bit Rate Codec (iLBC)", RFC 3951,
        December 2004.
   
   [42] Duric, A. and S. Andersen, "Real-time Transport Protocol (RTP)
        Payload Format for internet Low Bit Rate Codec (iLBC) Speech",
        RFC 3952, December 2004.
   
   [43] Herlein, G., et al., "RTP Payload Format for the Speex Codec",
        Work in Progress, October 2005.
   
   [44] TIA/EIA-810-A, "Transmission Requirements for Narrowband Voice
        over IP and Voice over PCM Digital Wireline Telephones", July
        2000.
   
   [45] TIA-EIA-IS-811, "Terminal Equipment - Performance and
        Interoperability Requirements for Voice-over-IP (VoIP) Feature
        Telephones", July 2000.
   
   [46] Alvestrand, H., "Tags for the Identification of Languages", BCP
        47, RFC 3066, January 2001.
   
   [47] Wing, D., "Symmetric RTP and RTCP Considered Helpful", Work in
        Progress, October 2004.
   
   [48] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN -
        Simple Traversal of User Datagram Protocol (UDP) Through Network
        Address Translators (NATs)", RFC 3489, March 2003.
   
   [49] Jennings, C., "NAT Classification Test Results", Work in
        Progress, July 2005.
   
   [50] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
        Methodology for Network Address Translator (NAT) Traversal for
        Offer/Answer Protocols", Work in Progress, July 2005.
   
   [51] Boulton, C. and J. Rosenberg, "Best Current Practices for NAT
        Traversal for SIP", Work in Progress, October 2005.
   
   [52] P. Eggert, "Sources for time zone and daylight saving time
        data." Available at http://www.twinsun.com/tz/tz-link.htm.
   
   [53] Campbell, B. and R. Sparks, "Control of Service Context using
        SIP Request-URI", RFC 3087, April 2001.
   
   [54] Mahy, R., "Connection Reuse in the Session Initiation Protocol
        (SIP)", Work in Progress, February 2006.
   
   [55] Jennings, C. and R. Mahy, "Managing Client Initiated Connections
        in the Session Initiation Protocol", Work in Progress, March
        2006.
   
   [56] Petrie, D., "A Framework for SIP User Agent Profile Delivery",
        Work in Progress, July 2005.
   
   [57] Jennings, C., "SIP Tutorial: SIP Security", presented at the VON
        Spring 2004 conference, March 29, 2004, Santa Clara, CA.
   
   [58] Jennings, C., Peterson, J., and M. Watson, "Private Extensions
        to the Session Initiation Protocol (SIP) for Asserted Identity
        within Trusted Networks", RFC 3325, November 2002.
   
   [59] Chokhani, S., Ford, W., Sabett, R., Merrill, C., and S. Wu,
        "Internet X.509 Public Key Infrastructure Certificate Policy and
        Certification Practices Framework", RFC 3647, November 2003.
   
   [60] Jennings, C. and J. Peterson, "Certificate Management Service
        for The Session Initiation Protocol (SIP)", Work in Progress,
        March 2006.
   
   [61] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
        (S/MIME) Version 3.1 Message Specification", RFC 3851, July
        2004.

[62] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.

Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004.

   [63] Audet, F. and C. Jennings, "NAT Behavioral Requirements for
        Unicast UDP", Work in Progress, September 2005.
   
   [64] Jennings, C., "Example call flows using SIP security
        mechanisms", Work in Progress, February 2006.
   
   [65] Peterson, J., "Session Initiation Protocol (SIP) Authenticated
        Identity Body (AIB) Format", RFC 3893, September 2004.

Author's Addresses

Henry Sinnreich
Pulver.com
115 Broadhollow Road
Melville, NY 11747, USA

   EMail: henry@pulver.com
   Phone: +1-631-961-8950

Steven Lass
Verizon
1201 East Arapaho Road
Richardson, TX 75081, USA

   EMail: steven.lass@verizonbusiness.com
   Phone: +1-972-728-2363

Christian Stredicke
snom technology AG
Gradestrasse, 46
D-12347 Berlin, Germany

   EMail: cs@snom.de
   Phone: +49(30)39833-0

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