Media Gateway Control Protocol Voiceband Data Package and General Purpose Media Descriptor Parameter Package

The term voiceband data (or simply VBD) refers to the use of a suitable voiceband codec (commonly G.711u or G.711a) for the transport of data payloads using the RTP protocol defined in IETF RFC 3550 [RFC3550]. This document defines Media Gateway Control Protocol (MGCP) [RFC3435] packages that enable a Call Agent to authorize and monitor the transition of a connection to and from VBD with or without redundancy [RFC2198] and FEC (forward error correction) [RFC5109]. There are a number of different VBD procedures. These procedures vary in terms of how the transition to and from VBD is coordinated end to end. Some coordination techniques are mutually negotiated by the two Gateways using the SDP [RFC4566]. These coordination techniques include, ITU-T Recommendation V.150.1 State Signalling Event (SSE) [V1501] ITU-T Recommendation V.152 Payload Type Switching [V152] Other coordination techniques are not negotiated. For example, the detection of fax, modem and text tones in the direction from the IP to the General Switched Telephone Network (GSTN) MAY result in a switch to VBD or a change (e.g., disable echo cancellation) to the Gateway controlled VBD procedure already in place. The IP-side detected tone serves as both a VBD stimulus and a coordination technique. IETF RFC 4733 [RFC4733] and RFC 4734 [RFC4734] can be used to convey fax and modem events and tones. As with IP-side tone detection, the telephone event MAY serve as both a VBD stimulus and a coordination technique. Note that while the use of RFC 4733 and RFC 4734 to convey fax and modem events and tones is negotiated, the use of RFC 4733 and RFC 4734 as a Gateway VBD coordination technique (at present) is not. The Voiceband Data (VBD) package is defined to support all VBD procedures. This document does not address the relative merits of different procedures nor advocate one procedure over another. We will use the term VBD to refer to Voiceband Data in general. In referring to VBD the package, we will use the term VBD package. We use the term "audio" (with double quotes) to refer to the IANA media type. We use the term audio (without double quotes) to refer to the use of the "audio" media type for (most commonly) voice. A package is defined for the General-Purpose Media Descriptor Parameter [V152]. In the context of VBD, the General-Purpose Media Descriptor Parameter (GPMD) package is used to authorize the negotiation of a particular codec for use with VBD. The General-Purpose Media Descriptor Parameter is "general" in nature and may be used in applications other than VBD. The Media Format Parameter (FM) package [RFC3660] describes the use of the standard audio MIME subtype "RED" in conjunction with the "fmtp" LocalConnectionOption in order to authorize the negotiation of redundancy [RFC2198], to identify the levels of redundancy and the media format associated with each redundancy level. This document will further explore the use of the FM package with VBD and redundancy. This document will explore the use of the FM package with VBD and FEC. The VBD package is intended to complement the MGCP Fax (FXR) package [RFCABCD]. This document will explore the use of the FXR package with VBD. The VBD package definition is provided in section 3. The GPMD package definition is provided in section 4. In section 5, we discuss the use of the FM package with VBD and redundancy. In section 6, we discuss the use of the FM package with VBD and FEC. In section 7, we discuss the use of the FXR package with VBD. In section 8, we provide two call flow examples showing how to use the VBD and GPMD packages. Security considerations are found in section 9, followed by the IANA considerations and references.

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.

A package is defined for (Voiceband Data) VBD. The package defines new events as detailed below. Package Name: VBD Package Version: 0

The following events are defined in support of the above:

The definitions of the individual events are provided in the following subsections.

The "Gateway Controlled Voiceband Data" (or simply "gwvbd") event occurs when a gwvbd procedure has been negotiated and VBD stimulus is detected. The "gwvbd" event may occur when the gwvbd procedure is updated (e.g., upon detecting new stimulus) and when the procedure fails. The "gwvbd" event occurs when the gwvbd procedure ends. The gwvbd procedure MUST be negotiated with the other side by passing and recognizing relevant parameters via the LocalConnectionDescriptor and RemoteConnectionDescriptor. The following recommendations from MGCP [RFC3435] apply.

The RequestedEvent is encoded as, GwVbdReqEvent = "gwvbd" The ObservedEvent is encoded as,

ABNF does not provide for position independent parameters. The "rc", "codec", "coord", and "dir" parameters, if present, MUST appear in the relative order shown. The "start", "update", "stop" and "failure" ObservedEvent parameters are defined: * VBD Start (start) The gwvbd procedure was initiated. The Call Agent SHOULD refrain from issuing media handling instructions to the Gateway until either a "gwvbd(stop)" or "gwvbd(failure)" event is generated. One and only one "gwvbd(stop)" or "gwvbd(failure)" event is generated corresponding to each "gwvbd(start)" event. * VBD Update (update) The gwvbd procedure was updated. The "gwvbd(update)" event MUST only be generated after a "gwvbd(start)" event and before a "gwvbd(stop)" or "gwvbd(failure)" event. * VBD Stop (stop) The gwvbd procedure ended and the Gateway did not detect any errors. Note that this does not necessarily imply a successful fax, modem, or text transmission. It merely indicates that the gwvbd procedure has ended and the procedure itself did not encounter any errors. The "stop" parameter may correspond to a change from VBD to a non-VBD "audio" codec or from VBD to another media type such as "image" or "text". This change may be under Call Agent or Gateway control. For example, the Gateway may coordinate the switch from VBD to "image/t38" through the exchange of SSEs [T38] and [V152]. For an example involving Call Agent control, refer to the "MC" Reason Code. In both examples, the gwvbd procedure ends with the media change. * VBD Failure (failure) The gwvbd procedure ended abnormally. Some kind of problem was encountered in the gwvbd procedure and the procedure ended. When the "gwvbd" event is reported, exactly one of the "start", "update", "stop", or "failure" parameters MUST be present and MUST be the first parameter supplied. The "rc", "codec", "coord" and "dir" ObservedEvent parameters are defined: * Coordination Technique (coord=<CoordinationTechnique>) The technique used to coordinate the transition to and from VBD with the remote endpoint. The Coordination Techniques are summarized in the following table:

With the "v152ptsw" Coordination Technique, payload type switching [V152] is used to coordinate the transition to and from VBD. With the "v150fw" Coordination Technique, state signalling events [V1501] are used to coordinate the transition to and from VBD. The list of Coordination Techniques may be extended to include values with meaning mutually understood between the Gateway and the Call Agent. * Reason Code (rc=<ReasonCode>) With the "start" and "update" parameter, the reason for triggering the switch/change to VBD. With the "stop" and "failure" parameter, the reason for triggering the switch from VBD. The Reason Codes in the following table, which are based on the ITU-T FAX/Textphone/Modem Tones Detection package [H2482], ITU-T V.150.1 Amendment 1 [V1501A1] and ITU-T V.152 [V152], may be used with the "start" and "update" parameter:

For solutions involving textphones using a modulation with interspersed text and speech on the same "channel" such as Baudot and EDT, the Call Agent SHOULD interpret the ReasonCode parameter as part of the "vbd/gwvbd(start)" event in order to differentiate between fax, modem and text. In the case of interspersed text and speech the Call Agent SHOULD remove the notification request for "vbd/gwvbd" upon receiving the "vbd/gwvbd(start)" event in order to avoid large numbers of notifications. For example, vbd/gwvbd(start, rc=Baudot) With a ReasonCode of "PTSW", the Call Agent cannot differentiate text from fax/modem. In this case, the Call Agent SHOULD adopt a policy which guards against large numbers of notifications. We consider several such policies. The Call Agent MAY remove the notification request for "vbd/gwvbd" upon receiving the "vbd/gwvbd(start, rc=PTSW)" event. With this policy, "update", "stop" and "failure" notifications will not be generated with text AND fax/modem. The Call Agent MAY wait for a subsequent "vbd/gwvbd(update)" event which differentiates text from fax/modem. If the ReasonCode indicates interspersed text and speech, the Call Agent SHOULD remove the notification request for "vbd/gwvbd". For example, vbd/gwvbd(update, rc=Edt) The Call Agent MAY remove the notification request for "vbd/gwvbd" upon receiving a "vbd/gwvbd(stop)" event without having differentiated between text and fax/modem. The Call Agent MAY remove the notification request for "vbd/gwvbd" after having received a number of "vbd/gwvbd(start)" events without having differentiated between text and fax/modem. The specific number of events after which the notification request is removed is considered an implementation detail outside the scope of this specification. Reason Codes applicable with the "stop" parameter:

The "MC" Reason Code indicates that the media type has changed from "audio" (to "image", "text", ...) or the "audio" media format has changed from a VBD codec (for a reason other than "PTSW"). For example, the gwvbd procedure may be initiated upon detecting CED. Subsequently, the Call Agent controlled T.38 procedure of the MGCP Fax (FXR) package [RFCABCD] may be initiated upon detecting V.21 flags. Upon receipt of a "t38(start)" event, the Call Agent will instruct the Gateway to switch from VBD to T.38 through the use of a ModifyConnection command involving a LocalConnectionOption encoding method of "L:a:image/t38" and/or a RemoteConnectionDescriptor with an "image/t38" media description. This stops the gwvbd procedure. There is no specific interdependency between the VBD package and the FXR package (or any other package). The gwvbd procedure is stopped as a consequence of the media change, not as a direct consequence of the T.38 procedure being initiated. Note that in this situation the "t38(start)" event will be sent before the "gwvbd(stop)" event. The Call Agent MAY choose to infer that the gwvbd procedure has ended upon receiving the "t38(start)" event and disable the notification of the "gwvbd" event. Refer to the example call flow in section 8.2. Reason Codes applicable with the "failure" parameter:

The list of Reason Codes may be extended to include values with meaning mutually understood between the Gateway and the Call Agent. * Codec String (codec=<CodecString>) With the "start" and "update" parameter, the codec parameter describes the MIME type associated with the switch/change to VBD (e.g., "audio/RED", "audio/PCMU", "audio/PCMA", "audio/G726-32", "audio/clearmode", ...). With the "stop" and "failure" parameter, the codec parameter describes the MIME type associated with the switch from VBD (e.g., "audio/G729", "image/t38", "text/t140", "audio/v150mr", ...). These strings should be full MIME types as listed in, http://www.iana.org/assignments/media-types * Direction of Stimulus (dir=<Direction>) With the "start" and "update" parameter, the "dir" parameter describes the direction of the stimulus which resulted in the switch/change to VBD.

Call Agents and Gateways MUST implement the "start" and "stop" parameters and MAY implement the "update" and "failure" parameters. Call Agents and Gateways MAY implement the "coord", "codec", and "dir" parameters. Call Agents MAY and Gateways MUST implement the "rc" parameter in conjunction with the "start" and "update" parameters. Call Agents and Gateways MAY implement the "rc" parameter in conjunction with the "stop" and "failure" parameters. A Call Agent MUST ignore all unknown ObservedEvent parameters including parameters which are defined as part of this specification and not implemented.

The following examples illustrate the encoding of the "gwvbd(start)" event,

The following example illustrates the encoding of the "gwvbd(update)" event,

The following examples illustrate the encoding of the "gwvbd(stop)" event,

The following examples illustrate the encoding of the "gwvbd(failure)" event,

The "No Negotiated Procedure for Voiceband Data" (or simply "nopvbd") event occurs when a VBD procedure has not been negotiated and VBD stimulus is detected. The "nopvbd" event may occur when the procedure is updated (e.g., upon detecting new stimulus), when the procedure ends and when the procedure fails. The nopvbd procedure MAY involve VBD handling including but not limited to adjusting gain and jitter, disabling voice activity detection and DC offset filters. The nopvbd procedure MAY involve switching to another codec. The Call Agent MAY have to issue further commands in response to the "nopvbd" event in order to ensure a successful VBD call. As with the "gwvbd" event, the same recommendations from MGCP [RFC3435] regarding ABNF, general robustness principles and white space apply.

The following ABNF notation is common with the "gwvbd" ObservedEvent.

ABNF does not provide for position independent parameters. The "rc", "codec", and "dir" parameters, if present, MUST appear in the relative order shown. The "start", "update", "stop" and "failure" ObservedEvent parameters are defined: * VBD Start(start) The nopvbd procedure was initiated. The Call Agent may have to issue further commands in order to ensure a successful VBD call (e.g., switch to another codec). At most one "nopvbd(stop)" or "nopvbd(failure)" event MAY be generated corresponding to each "nopvbd(start)" event. The Call Agent MAY need to infer that the nopvbd procedure has ended. * VBD Update (update) The nopvbd procedure was updated. The "nopvbd(update)" event MUST only be generated after a "nopvbd(start)" event and before a "nopvbd(stop)" or "nopvbd(failure)" event. * VBD Stop (stop) The nopvbd procedure ended and the Gateway did not detect any errors. Note that this does not necessarily imply a successful fax, modem, or text transmission. It merely indicates that the nopvbd procedure has ended and the procedure itself did not encounter any errors. Refer to the definition of the "stop" parameter from the "gwvbd" event in section 3.1.1 for additional information. * VBD Failure (failure) The nopvbd procedure ended abnormally. Some kind of problem was encountered in the nopvbd procedure and the procedure ended. Call Agents and Gateways MUST implement the "start" parameter and MAY implement the "update", "stop" and "failure" parameters. Call Agents MAY and Gateways MUST implement the "rc" parameter in conjunction with the "start" and "update" parameters. Call Agents and Gateways MAY implement the "rc" parameter in conjunction with the "stop" and "failure" parameters. A Call Agent MUST ignore all unknown ObservedEvent parameters including parameters which are defined as part of this specification and not implemented. The definitions of the "rc", "codec" and "dir" ObservedEvent parameters are taken from the "gwvbd" event. As with the "gwvbd" event, the same recommendations regarding interspersed text and speech apply.

The following examples illustrate the encoding of the "nopvbd(start)" event,

The following example illustrates the encoding of the "nopvbd(update)" event,

The following examples illustrate the encoding of the "nopvbd(stop)" event,

The following examples illustrate the encoding of the "nopvbd(failure)" event,

A package is defined for the General-Purpose Media Descriptor Parameter [V152]. The package defines a new LocalConnectionOption as detailed below.

The following LocalConnectionOption is defined in support of the above:

The definition of the LocalConnectionOption is provided in the following subsection.

The General-Purpose Media Descriptor Parameter LocalConnectionOption is similar to the "gpmd" SDP [RFC4566] attribute defined in ITU-T Recommendation V.152 [V152] and is applicable to all of the same media formats that the corresponding SDP "gpmd" attribute could be used with. The General-Purpose Media Descriptor Parameter is encoded as the keyword "gpmd" or "o-gpmd", followed by a colon and a quoted string beginning with the media format name (MIME subtype only) followed by a space, followed by the media format parameters associated with that media format,

" ]]> For simplicity, we will use the terms "codec" and "media format" interchangeably in the following. Multiple media formats may be indicated by either repeating the "gpmd" LocalConnectionOption multiple times, such as:

or alternatively by having a single "gpmd" keyword followed by a colon, and a semi-colon separated list of quoted strings for each General-Purpose Media Descriptor Parameter, as in:

The two formats may be mixed,

The carriage returns above are included for formatting reasons only and are not permissible in a real implementation. This holds true for all of the examples in this document. If it is possible for the same codec to be requested with and without the "gpmd" parameter, the following could result:

However, it would not be clear if the "gpmd" parameter was to be applied to the first or the second occurrence of the codec. The problem is that codec ordering is important (i.e., codecs are listed in preferred order), and the above syntax does not provide a way to indicate if "parameterX" is preferred (i.e., associated with the first "codec1") or not (i.e., associated with the second "codec1"). In order to resolve this dilemma, the codec in the "gpmd" media format is followed by a colon and an <order>, where <order> is a number from one to N for occurrences of the same codec in the codec list i.e.:

indicates that "parameterX" is associated with the second instance of "codec1" in the "a:codec1;codec1" list. If an invalid instance number is supplied (e.g., instance 3 where there are only two instances), then error code 524 - inconsistency in local connection options will be returned. In the absence of an <order>, the first instance is assumed. Pre-pending "gpmd" with the string "o-" (i.e., "o-gpmd") indicates that the parameter is optional. In that case, the Gateway may decide not to use the "gpmd" parameter specified, or only use it in part. If the "gpmd" LocalConnectionOption parameter is not optional (i.e., does not have "o-" in front of it), and the LocalConnectionOption parameter value is either not recognized or not supported, then the associated codec is considered "not supported". When auditing capabilities, the "gpmd" LocalConnectionOption parameter MUST be returned with a semi-colon separated list of supported formats and/or multiple independent "gpmd" parameters as in:

One example uses the General-Purpose Media Descriptor Parameter LocalConnectionOption in conjunction with Gateway controlled Voiceband Data (or simply VBD) using payload type switching [V152]. In the context of VBD, the <format> must be an RTP/AVP payload type. The <parameter list> is a semicolon-separated list of "parameter=value" pairs,

In the example below, G.729 is an audio codec and G.711u is a VBD codec,

The corresponding media description in the SDP as part of the connection request acknowledgment might look like:

If a request is made to audit the capabilities of an endpoint, and the endpoint supports G.711u as both an audio and VBD codec, then the "gpmd" LocalConnectionOption parameter might look like:

Given that some parameters, such as e.g., silence suppression, are only compatible with G.711u as an audio codec, then the Gateway MUST return different capability sets corresponding to audio and VBD. If we combine V.152 and redundancy [RFC2198], an example LocalConnectionOption might look as follows. In the example below, G.729 is an audio codec and G.711u is a VBD codec with a redundancy level of one:

The corresponding media description in the SDP as part of the connection request acknowledgment might look like:

Refer to section 5 for more examples involving V.152 and redundancy.

The MGCP Media Format Parameter (FM) package [RFC3660] in conjunction with the standard audio MIME subtype "RED" may be used by the Call Agent to authorize the negotiation of redundancy [RFC2198], to identify the levels of redundancy and the media format associated with each redundancy level. An example of this was demonstrated in section 4. The FM package states that the "fmtp" LocalConnectionOption MUST be returned when auditing capabilities. Applying this to VBD and redundancy might result in:

indicates that "formatX" is associated with the second instance of "codec1" in the "a:codec1;codec1" list. The examples in the FM package are limited to the use of the instance syntax in conjunction with the media format. We propose the use of the instance syntax in conjunction with the media format parameters,

Let's build on the example of section 4. In the example below, G.729 is an audio codec, G.711u is both an audio codec and a VBD codec with a redundancy level of one:

The corresponding media description in the SDP as part of the connection request acknowledgment might look like:

Note that the relative preference of the LocalConnectionOption encoding methods is preserved in the "audio" media formats (i.e., payload types) as part of the media description. In this example, this reflects a preference for V.152 with redundancy versus without. No preference is inferred from the relative order of the different LocalConnectionOptions, namely "a", "gpmd/gpmd" and "fmtp". A Call Agent can authorize the negotiation of audio codecs and VBD codecs involving different levels of redundancy. In the example below, G.711u is a VBD codec with a redundancy level of two (preferred) or one:

The corresponding media description in the SDP as part of the connection request acknowledgment might look like: �

Redundancy can be applied to both audio codecs and VBD codecs. In the example below, G.729 is an audio codec with a redundancy level of two and G.711u is a VBD codec with a redundancy level of one:

The corresponding media description in the SDP as part of the connection request acknowledgment might look like: �

A Call Agent may authorize the negotiation of forward error correction (FEC) [RFC5109] with the standard audio MIME subtype "parityfec",

By default, we assume that FEC packets are to be sent as a separate stream. The corresponding media description in the SDP as part of the connection request acknowledgment might look like:

If FEC is to be sent as a secondary codec in the redundant codec payload format [RFC2198], we again leverage the MGCP Media Format Parameter (FM) package [RFC3660] in�conjunction with the standard audio MIME subtype "RED",

The FM package states that the "fmtp" LocalConnectionOption MUST be returned when auditing capabilities. Applying this to VBD, redundancy and FEC might result in:

The MGCP Fax (FXR) package [RFCABCD] is used by a Call Agent to authorize fax handling including Call Agent controlled T.38 and Gateway procedures such as V.152. With the FXR package, VBD falls into one of two categories: "special fax handling" as part of the Gateway procedure (resulting in the "gwfax" event); or "no special fax handling" as part of the Gateway and Off procedures (resulting in the "nopfax" event). In order for a VBD procedure to fall into the "special fax handling" category, support for it MUST be negotiated with the other side by passing and recognizing relevant parameters via the LocalConnectionDescriptor and RemoteConnectionDescriptor. A Gateway controlled VBD procedure such as V.152 MUST fall into the category of Gateway controlled mode involving "special fax handling". The resulting "gwfax" event is what informs the Call Agent to refrain from issuing media handling instructions which could otherwise have a negative impact on the Gateway procedure. Consider the following example (with shorthand SDP notation):

The RemoteConnectionDescriptor does not indicate support for "image/t38" as a latent capability [RFC3407]. Consequently, the Gateway will not initiate the T.38 strict fax procedure, "t38", upon detecting fax stimulus (i.e., CNG, V.21 flags, ...). However, the two endpoints did successfully negotiate a Gateway controlled VBD procedure (e.g., V.152), therefore, Gateway controlled mode involving "special fax handling" is used. The "gwfax(start)" event will be generated upon detecting VBD (including fax) stimulus. A Call Agent can express a preference for a Gateway procedure involving "special fax handling" over a T.38 procedure (strict or loose). For example, L: fxr/fx:gw;t38 and, L: fxr/fx:gw;t38-loose However, with the existing syntax of the FXR package, a Call Agent cannot express a preference for one Gateway procedure over another, each with possibly different preferences relative to a T.38 procedure. The FXR package allows a Gateway to implement additional fax handling parameters. We define just such a parameter by qualifying the existing "gw" parameter with a list of one or more MIME types,

By qualifying the "gw" parameter with a list of MIME types, we narrow the scope of the Gateway procedure. Consider the following examples in which the Call Agent authorizes the use of a Gateway controlled fax handling procedure:

Only "special fax handling" involving one of the specified MIME types is authorized. Support for "special fax handling" involving one of the specified MIME types MUST be negotiated or this "instance" of the Gateway procedure is not initiated. Consider the following example in which the Call Agent authorizes the use of a Gateway controlled fax handling procedure:

In this example, the call will fail if the Gateway fails to negotiate "audio/t38". The "fx" LocalConnectionOption MAY now involve multiple instances of the "gw" parameter, each with a different list of MIME types. In order to authorize "no special fax handling", the Call Agent MUST include: the "gw" parameter without a MIME type; or the "off" parameter. The instance of the "gw" parameter without a MIME type should appear as the last instance of the "gw" parameter. In the following example, L: a:G729;PCMU, fxr/fx:gw[image/t38];gw the Call Agent authorizes the use of and expresses a preference for, 1. Gateway controlled image/t38 (e.g., T.38oUDPTL) 2. Any other Gateway procedure with "special fax handling" 3. No special fax handling If present, the "off" parameter should appear as the last parameter. In the following example, L: a:G729;PCMU;t38, fxr/fx:gw[audio/t38];off the Call Agent authorizes the use of and expresses a preference for, 1. Gateway controlled audio/t38 (e.g., T.38oRTP) 2. No special fax handling We can express relative preferences for different Gateway controlled fax handling procedures, not only with respect to one another, but with respect to T.38 procedures. Consider the following preferential list of fax handling procedures, 1. Gateway controlled audio/t38 (e.g., T.38oRTP) 2. Gateway controlled image/t38 (e.g., T.38oUDPTL) 3. Call Agent Controlled image/t38 4. Gateway controlled VBD with Redundancy (e.g., PCMU, V.152 and RFC 2198) 5. Gateway controlled VBD without Redundancy (e.g., PCMU and V.152) 6. Any other Gateway procedure with "special fax handling" 7. No special fax handling This would be expressed as,

Note that the bracketed form of the "gw" parameter is NOT defined as part of the VBD package. The bracketed form of the "gw" parameter is defined as an extension to the FXR package. Gateways that implement the bracketed form of the "gw" parameter MUST return this form of the parameter when capabilities are audited as illustrated by the following example:

Support for the bracketed "gw" parameter MAY be spread across multiple capability lines,

A Call Agent SHOULD only attempt to leverage the bracketed form of the "gw" parameter in conjunction with an endpoint which indicates support for the bracketed syntax as part of its capabilities. Call Agents and Gateways that do not support this form of the "gw" parameter MUST ignore the bracketed MIME type information consistent with the MGCP grammar [RFC3435].

In this section, we provide two example call flows. The first one illustrates a modem call under Gateway control using V.152. The second one illustrates a fax call under Gateway control using V.152 and Call Agent controlled T.38.

In this example, both sides support Gateway controlled VBD using V.152 with redundancy. We assume the originating and terminating Call Agents communicate via the Session Initiation Protocol (SIP) [RFC3261]:

Step 2: The Gateway acknowledges the command and includes SDP with codec information as well as V.152 and redundancy information:

Step 3: The originating Call Agent sends a SIP INVITE message with the SDP to the terminating Call Agent. Step 4: The terminating Call Agent issues a CreateConnection command to the terminating Gateway instructing it to use G.729 media encoding and to notify it of the "gwvbd" and "nopvbd" events. Again, the Call Agent authorizes the negotiation of G.711u as a VBD codec with a redundancy level of one:

Step 5: The terminating Gateway supports V.152 and redundancy, and the RemoteConnectionDescriptor included indicates that the other side supports V.152 and redundancy. The terminating Gateway sends back a success response with its SDP which also includes V.152 and redundancy information:

Step 6: The terminating Call Agent sends back a SIP 200 OK response to the originating Call Agent, which in turn sends a SIP ACK (not shown). Step 7: The originating Call Agent in turns sends a ModifyConnection command to the originating Gateway:

Since the RemoteConnectionDescriptor indicates the other side supports V.152 and redundancy, the Gateway will in fact be able to use the Gateway controlled VBD procedure with redundancy. Had there not been any support for V.152 in the RemoteConnectionDescriptor, then this command would still have succeeded, however there would be no negotiated procedure for VBD handling. Step 8: The Gateway acknowledges the command. At this point, a call is established using G.729 encoding, and if a VBD call is detected, the Gateway controlled VBD procedure will be initiated. Step 9-10: A modem call now occurs. The terminating Gateway detects a T.30 CED tone (a.k.a. V.25 ANS) in the GSTN to IP direction and begins transmitting RTP packets with the negotiated redundant VBD payload type (96). The "gwvbd(start)" event occurs and is notified to the Call Agent:

Step 11: The Call Agent acknowledges the Notify command: 200 2500 OK Step 12: Upon receiving a RTP packet with the redundant VBD payload type (96), the originating Gateway begins transmitting RTP packets with the redundant VBD payload type. The "gwvbd(start)" event occurs and is notified to the Call Agent:

Step 13: The Call Agent acknowledges the Notify command: 200 1500 OK Step 14 - 15: The modem call ends. The terminating Gateway detects bi-directional silence and begins transmitting RTP packets with the negotiated audio payload type (18). The "gwvbd(stop)" event occurs and is notified to the Call Agent:

Step 16: The Call Agent acknowledges the Notify command: 200 2501 OK Step 17: Upon receiving a RTP packet with the audio payload type (18), the originating Gateway begins transmitting RTP packets with the audio payload type. The "gwvbd(stop)" event occurs and is notified to the Call Agent:

Step 18: The Call Agent acknowledges the Notify command:

The modem call is now over.

In this example, both sides support Gateway controlled VBD using V.152 with Redundancy and Call Agent controlled T.38. We assume the originating and terminating Call Agent communicate via the Session Initiation Protocol (SIP) [RFC3261]:

| | | | 3| | INVITE(sdp-o)|-> | | | 4| | | CRCX(sdp-o)|-> | | 5| | | <-|200 (sdp-t) | | 6| | <-|200(sdp-t) | | | 7| <-|MDCX(sdp-t) | | | | 8| 200|-> | | | |--|---------------|---------------|---------------|---------------| | 9| | | |<- ANS/T.30 CED| |10| | | <- NTFY(gwvbd start)| |11| | | 200|-> | |12|NTFY(gwvbd start) -> | | | |13| <-|200 | | | |14| | | <- V.21 Preamble| |15| | | <- NTFY(t38 start)| |16| | | 200|-> | |17| | | MDCX(t38)|-> | |18| | | <-|200(sdp-t2) | |19| | <-|INVITE(sdp-t2) | | |20| <-|MDCX(sdp-t2) | | | |21| 200(sdp-o2)|-> | | | |22| | 200(sdp-o2)|-> | | |23| | | MDCX(sdp-o2)|-> | |24| | | <-|200 | |25| V.21 Preamble |-> | | | |26|NTFY(t38 start)|-> | | | |27| <-|200 | | | |--|---------------|---------------|---------------|---------------| |28| | | | (fax ends) | |29| | | <-|NTFY(t38 stop) | |30| | | 200|-> | |31|NTFY(t38 stop) |-> | | | |32| <-|200 | | | ------------------------------------------------------------------ ]]> Step 1: The Call Agent issues a CreateConnection command to the Gateway instructing it to use G.729 media encoding and to use either the strict T.38 procedure or the Gateway procedure. Consequently, the Call Agent requests notification of the "t38", "gwfax", "gwvbd" and "nopvbd" events. The Call Agent authorizes the negotiation of G.711u as a VBD codec with a redundancy level of one:

Step 2: The Gateway acknowledges the command and includes SDP with codec information as well as capability, V.152 and redundancy information:

Note that V.152 requires the use of the session-level "a=pmft" SDP attribute in order to express a preference for T.38 over V.152 for fax handling. Step 3: The originating Call Agent sends a SIP INVITE message with the SDP to the terminating Call Agent. Step 4: The terminating Call Agent issues a CreateConnection command to the terminating Gateway instructing it to use G.729 media encoding and to use either the strict T.38 procedure or the Gateway procedure. Consequently, the Call Agent requests notification of the "t38", "gwfax", "gwvbd" and "nopvbd" events. Again, the Call Agent authorizes the negotiation of G.711u as a VBD codec with a redundancy level of one:

Step 5: The terminating Gateway supports T.38, and the RemoteConnectionDescriptor included indicates that the other side supports T.38 as well, so the strict T.38 Call Agent controlled procedure requested can be used. The terminating Gateway supports V.152 and redundancy, and the RemoteConnectionDescriptor included indicates that the other side supports V.152 and redundancy, so Gateway controlled VBD using V.152 and redundancy can be used for modem and text transmissions. The terminating Gateway sends back a success response with its SDP which also includes capability, V.152 and redundancy information:

The ModifyConnection command does not repeat the LocalConnectionOptions sent previously. As far as fax handling is concerned, the Gateway therefore attempts to continue using the current fax handling procedure, i.e. strict Call Agent controlled T.38. Since the capability information indicates the other side supports T.38, the Gateway will in fact be able to use the strict Call Agent controlled T.38 procedure. Since the RemoteConnectionDescriptor indicates the other side supports V.152 and redundancy, the Gateway will in fact be able to use the V.152 VBD procedure with redundancy. Step 8: The Gateway acknowledges the command. At this point, a call is established using G.729 encoding, and if a fax call is detected, the Call Agent controlled T.38 procedure will be initiated. If a modem or text call is detected, the V.152 VBD procedure will be initiated. Step 9-10: The terminating Gateway detects the T.30 CED tone (a.k.a. V.25 ANS). Since both fax and modem calls can start with this sequence, it is not possible to determine that this is a fax call until step 14, where the V.21 fax preamble is detected. The terminating Gateway begins transmitting RTP packets with the negotiated redundant VBD payload type (96). The "gwvbd(start)" event occurs and is notified to the Call Agent:

Step 11: The Call Agent acknowledges the Notify command:

Step 12: Upon receiving a RTP packet with the redundant VBD payload type (96), the originating Gateway begins transmitting RTP packets with the redundant VBD payload type. The "gwvbd(start)" event occurs and is notified to the Call Agent:

Step 13: The Call Agent acknowledges the Notify command:

Step 14 - 15: The terminating Gateway detects the V.21 fax preamble. The terminating Gateway is using the Call Agent controlled T.38 strict procedure for fax calls, so the "t38(start)" event occurs:

Step 16: The Call Agent acknowledges the Notify command:

Step 17: The Call Agent then instructs the terminating Gateway to change to using the "image/t38" MIME type instead:

Note that the Call Agent is no longer requesting notification of the "gwvbd" event. Step 18: The terminating Gateway sends back a success response with its SDP which also includes the "image/t38" media description:

The gwvbd procedure ends due to the media type change. The "gwvbd(stop)" event would normally be notified at this point, however, the Call Agent is no longer requesting notification of the "gwvbd" event. The Call Agent would have inferred from the "t38(start)" event that the gwvbd procedure ended. Step 19: The terminating Call Agent sends a re-INVITE to the originating Call Agent with the updated SDP. Step 20: The originating Call Agent then sends a ModifyConnection command to the originating Gateway:

Step 21: The originating Gateway changes to T.38 and sends back a success response with updated SDP:

Again, the gwvbd procedure ends due to the media type change. The "gwvbd(stop)" event would normally be notified at this point, however, the Call Agent is no longer requesting notification of the "gwvbd" event. Step 22: The originating Call Agent sends a SIP 200 OK response with the updated SDP to the terminating Call Agent, which in turn sends a SIP ACK (not shown). Step 23: The terminating Call Agent sends a ModifyConnection with the updated SDP to the terminating Gateway:

Step 24 - 32: These steps correspond to the Call Agent controlled T.38 strict procedure as defined in the MGCP Fax (FXR) package [RFCABCD].

The MGCP VBD package itself is not known to introduce any new security concerns. VBD media is transported over RTP. If security services are in place to protect RTP media streams, these will also be in effect for the VBD media stream. The MGCP GPMD package does not require any further security considerations beyond those indicated in MGCP [RFC3435].

The IANA is hereby requested to register the following MGCP packages,

and the following LocalConnectionOption,

Several people have contributed to the development of the MGCP VBD and GPMD packages and the use of the MIME subtypes "RED" and "parityfec" with the FM package for VBD with redundancy and FEC. In particular, the authors would like to thank Flemming Andreasen, John Atkinson, Bill Foster and the CableLabs PacketCable TGCP/NCS focus team for their contributions. Many thanks to Billy Hare for doing a thorough review of this document.

Changes in 01: * Number of formatting corrections (mainly to remove extra blank lines) * Updated a number of internal section references (these were off by one due to addition of new section 2 Terminology as part of initial version 00 submission) * In section 3.1.2, corrected Start VBD to VBD Start * In section 4.1.1, added text to clarify that the first instance is assumed in the absence of <order> Changes in 02: * Fix errors, warnings and comments reported by ID nits Changes in 03: * Editorial corrections throughout the document Changes in 04: * No changes