Internet Engineering Task Force F. Andreasen Internet Draft Cisco Systems Document: draft-andreasen-mgcp-fax-03.txt July 18, 2004 Category: Informational Media Gateway Control Protocol Fax Package Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on January 18, 2005. Abstract This document defines a Media Gateway Control Protocol (MGCP) package to support fax calls. The package allows for fax calls to be supported in two different ways. The first one utilizes ITU-T Recommendation T.38 for fax relay under the control of the Call Agent. The second one lets the gateway decide upon a method as well as handle the details of the fax call without Call Agent involvement. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC-2119 [RFC2119]. Andreasen Informational - Expires January 2005 1 MGCP Fax Package July 2004 Table of Contents 1. Introduction......................................................3 2. Fax Package Definition............................................3 2.1 LocalConnectionOptions..........................................3 2.1.1 T.38 Procedure (Strict or Loose)............................5 2.1.2 Gateway Procedure...........................................6 2.1.3 Off Procedure...............................................6 2.1.4 Mode Operation..............................................7 2.2 Events and Signals..............................................8 2.2.1 Gateway Controlled Fax (gwfax)..............................9 2.2.2 No Special Fax Handling (nopfax)............................9 2.2.3 T.38 fax relay(t38):.......................................10 2.3 Connection Parameters..........................................11 2.4 Media IP Address and Port for T.38.............................12 3. Call Flow Examples...............................................12 3.1 Call Agent Controlled T.38 Strict..............................12 3.2 Multiple and Different Options.................................18 4. Security Considerations..........................................25 5. IANA Considerations..............................................25 6. Normative References.............................................25 7. Informative References...........................................25 8. Acknowledgements.................................................26 9. Author's Address.................................................26 Andreasen Informational - Expires January 2005 2 MGCP Fax Package July 2004 1. Introduction This document defines a Media Gateway Control Protocol (MGCP) [RFC3435] package that enables MGCP controlled gateways to support fax calls. The package enables fax calls to be supported in two different ways. The first one utilizes ITU-T Recommendation T.38 using either UDPTL or TCP (see [T38]) for fax relay under the control of the Call Agent. The second one lets the gateway decide upon a method as well as handle the details of the fax call without Call Agent involvement. The fax package definition is provided in Section 2 and in Section 3 we provide two call flow examples showing how to use it. Security considerations are found in Section 4, followed by the IANA considerations and references. 2. Fax Package Definition A package is defined for fax. The package defines new localconnectionoptions, events, and connection parameters as detailed below. Package Name: FXR Package Version: 0 2.1 LocalConnectionOptions A new Fax LocalConnectionOptions (LCO) parameter is defined for fax handling. The Call Agent supplies this fax LCO to indicate the desired fax handling procedure to the Media Gateway. The fax LCO contains an ordered list of desired fax handling procedures. When the parameter is explicitly included in a command, the gateway MUST be able to use at least one of the listed procedures for the command to succeed. The list can currently contain one or more of the following values (see Section 2.1.1 to 2.1.4 for further details on these): * T.38 Strict Use T.38 [T38] with either UDPTL or TCP for fax relay and have the Call Agent control it. Assuming the procedure can be used (see Section 2.1.1), a switch to T.38 procedures will be initiated upon fax detection and a "t38(start)" event will be generated (see Section 2.2). This mode requires an indication of T.38 support from the remote side in order to be used, as described further in Section 2.1.1. * T.38 Loose Identical to T.38 Strict mode, except that an indication of T.38 support from the remote side is not required for the mode to be used. Andreasen Informational - Expires January 2005 3 MGCP Fax Package July 2004 * Off Do not invoke any special procedure for fax, except for echo cancellation adjustment and possibly switching to another codec. * Gateway Let the gateway control and decide how to handle fax calls without Call Agent involvement. This includes the case where the gateway does not do anything special for fax, hence by definition this procedure can always be supported. If the gateway does invoke a special procedure upon detection of fax, it will generate a "gwfax(start)" event so the Call Agent can be notified about it (see Section 2.2). The Call Agent SHOULD then refrain from issuing potentially conflicting commands to the gateway until the gateway ends its special fax handling procedure. A gateway that ends up not being able to invoke any special procedure for fax will generate a "nopfax(start)" event (see Section 2.2) upon detection of fax. The set of possible values for the fax LCO is extensible. The prefix "x-", which indicates an optional extension, and the prefix "x+", which indicates a mandatory extension, are reserved for vendor-specific use. In CreateConnection commands, the fax LCO value defaults to "gateway". In ModifyConnection commands, the fax LCO value defaults to its current value on the connection. Thus, if LocalConnectionOptions are either omitted or the fax LCO is not included in a ModifyConnection command, the previous fax LCO value for the connection will be retained, but without affecting the outcome of the command; consequently, the gateway may now not apply any special procedure to fax. If the Call Agent wants to ensure that a command succeeds only when a fax procedure is applied, the command needs to include the fax LCO explicitly. As an example of this, assume that the CreateConnection command successfully specified the use of "T.38 Strict", and a ModifyConnection command is now received without the fax LCO, but with a RemoteConnectionDescriptor indicating no support for T.38; in that case, the ModifyConnection will succeed, however T.38 procedures will no longer be invoked upon fax detection. Had the Call Agent instead included the fax LCO set to "T.38 Strict", the command would have failed. If multiple fax parameter values are provided, the gateway MUST choose one of the values. Please refer to Section 2.1.4 for further details. The fax LCO parameter is encoded as the keyword "fx" (prefixed with the package name per [RFC3435]), followed by a colon and a semicolon separated list of values where T.38 Strict is encoded as "t38", T.38 Loose is encoded as "t38-loose", gateway is encoded as "gw", and off is encoded as "off". Andreasen Informational - Expires January 2005 4 MGCP Fax Package July 2004 The following example illustrates use of PCMU or G.729 for audio encoding and T.38 Strict fax relay (preferred) or gateway control for fax: L: a:PCMU;G729, fxr/fx:t38;gw When auditing capabilities, the fax LCO may be returned with a semi- colon separated list of supported fax handling parameters. The values "t38", "off" and "gw" MAY be omitted from such a list as they are always implied. Gateways that implement additional parameters SHOULD return these additional parameters when capabilities are audited as illustrated by the following example: A: a:image/t38, fxr/fx:mypar, ... In the following subsections we provide additional detail on the above defined fax procedures. 2.1.1 T.38 Procedure (Strict or Loose) When a gateway is instructed to use one of the T.38 procedures, also known as Call Agent controlled T.38 mode, the "m=" line in the SDP returned will not indicate use of UDPTL-based or TCP-based T.38 (unless the gateway was also instructed to use "image/t38" for the media stream). Any other entity seeing this SDP will not know whether T.38 is supported or not and hence whether it is safe to attempt a switch to T.38 upon fax detection. To remedy this dilemma, capability information for T.38 (if supported) using the SDP Simple Capability Declaration extensions [RFC3407] SHOULD be included as illustrated in the following example - other capability information MAY be included as well: m=audio 3456 RTP/AVP 18 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 18 a=cdsc: 2 image udptl t38 For a list of T.38 related parameters to be included in the SDP, please refer to T.38 Annex D [T38]. Upon fax detection, a gateway which has successfully been instructed to use one of the T.38 procedures will: 1. Initiate the T.38 fax relay procedure and mute the media channel (unless the media channel is already using T.38). 2. Generate a "t38(start)" event. 3. Await further instructions from the Call Agent in order to initiate the actual media change. The Call Agent instructs the gateway to perform the media change by sending it a ModifyConnection command with "image/t38" listed as the Andreasen Informational - Expires January 2005 5 MGCP Fax Package July 2004 encoding method in the LocalConnectionOptions (receipt of a ModifyConnection command without LocalConnectionOptions but with a RemoteConnectionDescriptor containing an "m=" line with the MIME type "image/t38" would achieve the same). Per the normal MGCP codec negotiation procedures (see [RFC3435] Section 2.6), if a RemoteConnectionDescriptor was included as well, it needs to include an "m=" line with "image/t38" as an acceptable media format in order for the command to succeed. The gateway may choose between the UDPTL and TCP transport protocols at its own discretion subject to the normal MGCP codec negotiation procedures (in practice, TCP-based implementations are currently rare). If a RemoteConnectionDescriptor was not included with the ModifyConnection command sent to a gateway that initiated the T.38 procedure, it is possible (in fact likely), that the last received RemoteConnectionDescriptor did not include an "m=" line listing "image/t38" as an acceptable media format. In that case, the endpoint cannot send T.38 media to the other side. The endpoint MUST instead wait for an updated RemoteConnectionDescriptor with "image/t38" as an acceptable media format and a supported transport protocol (UDPTL or TCP). The T.38 fax procedure continues when an acceptable RemoteConnectionDescriptor is received; an acceptable RemoteConnectionDescriptor contains an "m=" line with the "image/t38" MIME type (using the normal SDP syntax) and a supported transport protocol (UDPTL or TCP). If the fax call fails, e.g., due to a fax timeout, while waiting for an acceptable RemoteConnectionDescriptor, a "t38(stop)" or a "t38(failure)" event will be generated. When the T.38 procedure ends, a "t38(stop)" or "t38(failure)" event will be generated. 2.1.2 Gateway Procedure A gateway using the gateway procedure, also known as Gateway controlled mode, may initiate special fax handling upon detecting a fax call. The details of this special fax handling are outside the scope of this document. However, in order to use any special fax handling, support for it MUST be negotiated with the other side by passing and recognizing relevant parameters via the RemoteConnectionDescriptor. If the other side has not indicated support for the special fax handling desired, the gateway MUST NOT attempt to initiate it. When special fax handling is initiated, a "gwfax(start)" event is generated thereby enabling the Call Agent to differ between the Call Agent and gateway controlled mode while still being informed about the actual change to fax. The special gateway handling of fax ends when a "gwfax(stop)" or "gwfax(failure)" event is generated. 2.1.3 Off Procedure A gateway using the "off" procedure will not invoke any special fax procedures, e.g. T.38, when detecting a fax. However, the gateway may still adjust local echo cancellation and/or switch to an Andreasen Informational - Expires January 2005 6 MGCP Fax Package July 2004 alternative codec as needed (in particular, this does not preclude the use of RTP-based T.38). Also, a "nopfax(start)" event will be generated; a corresponding "stop" event however will not. Generating a "stop" event would imply that the gateway had to infer when the fax call ends, which involves processing of the media stream. However, when using the "off" mode, such processing is not expected to occur. 2.1.4 Mode Operation For each of the above modes, the RemoteConnectionDescriptor provides information on what procedure(s) the other side supports. The following rules are used to determine which procedure to use: 1. Whatever the Call Agent specified in the Fax LocalConnectionOptions for the current command MUST be adhered to. If the gateway cannot satisfy any of the options, the command fails (error code 532 - unsupported value(s) in LocalConnectionOptions is RECOMMENDED). 2. If both Fax LocalConnectionOptions and a RemoteConnectionDescriptor are provided, the procedure selected MUST be supported by both sides - this is currently only an issue for "T.38 Strict". A procedure can be satisfied by the remote side if: * the relevant MIME media type, e.g. "image/t38", is included in the "m=" line in the RemoteConnectionDescriptor, or * the relevant MIME media type is included as a capability (see [RFC3407]) in the RemoteConnectionDescriptor. If the gateway cannot select any of the procedures in the Fax LocalConnectionOptions, the command fails (error code 532 is RECOMMENDED). Note that "T.38 Loose", "gateway", and "off" by definition can always be supported by an implementation that supports this package. 3. If the Call Agent did not include any Fax LocalConnectionOptions or a RemoteConnectionDescriptor with the command, the gateway MUST continue using whichever procedure it is currently using. 4. If the Call Agent did not include any Fax LocalConnectionOptions, but a RemoteConnectionDescriptor was included, the gateway follows rule 2 in selecting a procedure. In so doing, the default Fax LocalConnectionOptions, i.e. "gateway" in CreateConnection, or the current value in ModifyConnection, will be used. In the case of ModifyConnection, the outcome of the command does not depend on the gateway being able to select one of these "default" procedures (as described in Section 2.1). Note that this is not an issue for CreateConnection, since the default value can always be supported by definition. Andreasen Informational - Expires January 2005 7 MGCP Fax Package July 2004 5. A previously received RemoteConnectionDescriptor does not affect what procedure can be selected. Only a RemoteConnectionDescriptor supplied with the current command affects the procedure selection. However, in order to send media of a given type (e.g. "image/t38"), the most recently received RemoteConnectionDescriptor MUST include a corresponding media line. The following examples illustrate the use of the above rules: Per rule 1, a gateway that only supports standard T.38 fax relay will fail a command that only contains the fax option "mypar" whereas it will succeed a command that contains, "t38-loose", "gw", "off" or no Fax LCO. A command that only contained "t38", i.e. use of T.38 in "strict" mode, may or may not succeed (depending on the RemoteConnectionDescriptor). A gateway supporting T.38 that receives a CreateConnection command with the fax handling LCO set to "t38" and a RemoteConnectionDescriptor with neither a T.38 capability nor a T.38 media stream will fail per rule 2. Had the fax handling LCO included either "t38-loose", "gw" or "off", the command would have succeeded and any of the procedures included could have been selected. Assume a gateway supporting T.38 has successfully executed a CreateConnection command with fax handling set to "t38". If the gateway now receives a ModifyConnection command without a fax handling LCO but with a RemoteConnectionDescriptor that has neither a T.38 capability nor a media stream with "image/t38", the command will succeed (since rule 1 has no effect in that case). However, per rule 2 and 4, there will not be any T.38 procedure in place. Had the CA instead included a fax handling LCO set to "t38" again, the command would have failed per rule 2. Finally, it should be noted that a switch to T.38 can be initiated by either one or both of the originating and terminating gateways and hence implementations MUST be prepared to handle this. This includes the case where both sides initiate the switch, which for example can occur when the originating fax generates Calling Tone (CNG) and the terminating fax detects V.21 fax preamble (see [T30]) before the switch to T.38 has been performed on the terminating side. 2.2 Events and Signals The following events are defined in support of the above: Andreasen Informational - Expires January 2005 8 MGCP Fax Package July 2004 ------------------------------------------------------------------ | Symbol | Definition | R | S Duration | |---------|----------------------------|-----|---------------------| | gwfax | Gateway controlled fax | x | | | nopfax | No special fax handling | x | | | t38 | T.38 fax relay | x | | ------------------------------------------------------------------ The definitions of the individual events are provided in the following subsections. 2.2.1 Gateway Controlled Fax (gwfax) The "gateway controlled fax" event occurs when the gateway handled fax procedure either starts, stops or fails. The event is encoded as "gwfax" and the following event parameters, which apply to ObservedEvents only, are defined: * start Gateway controlled fax procedure was initiated. The Call Agent SHOULD refrain from issuing media handling instructions to the gateway until either a "gwfax(stop)" or "gwfax(failure)" event is generated. * stop Gateway controlled fax procedure ended and the gateway did not detect any errors. Note that this does not necessarily imply a successfully transmitted fax. It merely indicates that the gateway controlled fax procedure has ended and the procedure itself did not encounter any errors. Media parameters for the connection are as before the gateway handled fax procedure started. * failure The gateway controlled fax procedure ended abnormally. Some kind of problem was encountered in the gateway controlled fax procedure and the procedure ended. Media parameters are as before the gateway handled fax procedure started. One of the above parameters will be present when the event is reported. The "gwfax" event MAY be parameterized with additional parameters in ObservedEvents, however it is RECOMMENDED that one of the above parameters is the first parameter supplied. Unknown parameters MUST be ignored. The following example illustrates the encoding of the "gwfax" event: O: fxr/gwfax(start) O: fxr/gwfax(stop, foobar) 2.2.2 No Special Fax Handling (nopfax) Andreasen Informational - Expires January 2005 9 MGCP Fax Package July 2004 The "no special fax handling" event occurs when there is no special fax handling procedure in place and a fax call is detected. This can happen either due to no special fax handling procedure being requested (including "off"), or negotiation leading to no special fax handling procedure being possible. The event is encoded as "nopfax" and the following event parameter, which applies to ObservedEvents only, is defined: * start No special fax handling procedure is in place, however a fax call is now detected. The Call Agent may have to issue further commands in order to ensure a successful fax call (e.g., switch to another codec). The above parameter will be present when the event is reported. The "nopfax" event MAY be parameterized with additional parameters on ObservedEvents, however it is RECOMMENDED that the above parameter is the first parameter supplied. Unknown parameters MUST be ignored. Note, that this event currently cannot be parameterized with "stop" or "failure" as it only detects the beginning of a fax call. The following example illustrates the encoding of the "nopfax" event: O: fxr/nopfax(start) 2.2.3 T.38 fax relay(t38): The "T.38 fax relay" event occurs when one of the T.38 fax relay procedures (strict or loose) either starts, stops or fails. The event is encoded as "t38" and the following event parameters, which apply to ObservedEvents only, are defined: * start Call Agent controlled T.38 fax relay procedure was initiated. The Call Agent SHOULD modify each side of the connection to start using the "image/t38" media format, unless they already do. * stop Call Agent controlled T.38 fax relay procedure ended and the gateway did not detect any errors. Note that this does not necessarily imply a successfully transmitted fax. It merely indicates that the Call Agent controlled T.38 fax relay procedure has ended and the procedure itself did not encounter any errors. The Call Agent may want to modify the media parameters for each side of the connection. Note that, in contrast to the gateway controlled fax procedure case, media parameters such as codecs do not automatically revert to their values before the start of the fax call; echo cancellation and silence suppression however does per the procedures in [RFC3435] Section 2.3.5. * failure Call Agent controlled T.38 fax relay procedure ended abnormally. Some kind of problem in the Call Agent Andreasen Informational - Expires January 2005 10 MGCP Fax Package July 2004 controlled T.38 fax relay procedure was encountered and the procedure ended. The Call Agent may want to modify the media parameters for each side of the connection. Note that, in contrast to the gateway controlled fax procedure case, media parameters such as codecs do not automatically revert to their state before the start of the fax call; echo cancellation and silence suppression however does per the procedures in [RFC3435] Section 2.3.5. One of the above parameters will be present when the event is reported. The "t38" event MAY be parameterized with additional parameters, however it is RECOMMENDED that one of the above parameters is the first parameter supplied. Unknown parameters MUST be ignored. The following example illustrates the encoding of the "t38" event: O: fxr/t38(start) O: fxr/t38(foobar, stop) 2.3 Connection Parameters The connection parameters for the connection, that measures packets and octets sent and received, MUST include packets and octets for fax handling as well. Interarrival jitter and average transmission delay calculation however MAY not be performed while fax is in progress, e.g., if T.38 is used. In such cases, the interarrival jitter and average transmission delay calculations are simply suspended until calculations can resume, e.g., by changing back to an RTP-based media stream again. In addition to these connection parameters, the fax package defines the following connection parameters, which gateways MAY support: Number of fax pages sent (PGS): The cumulative number of fax pages sent by the endpoint for the life of the connection. The parameter is encoded as "PGS" and the value supplied is a string of up to nine decimal digits. Number of fax pages received (PGR): The cumulative number of fax pages sent by the endpoint for the life of the connection. The parameter is encoded as "PGR" and the value supplied is a string of up to nine decimal digits. The following example illustrates the use of these parameters: P: FXR/PGS=3, FXR/PGS=0, PS=1245, OS=62345, ... Andreasen Informational - Expires January 2005 11 MGCP Fax Package July 2004 2.4 Media IP Address and Port for T.38 When an endpoint is instructed to change to or from T.38 for a media stream, it SHOULD continue using the same IP address and port as the media stream is currently using, since this will minimize any Quality of Service, Network Address Translator (NAT) and Firewall interactions from the change. However, if an endpoint has a good reason, it MAY choose not to follow this recommendation. 3. Call Flow Examples In this section, we provide two example call flows. The first one illustrates a T.38 fax call under Call Agent control on both the originating and terminating side. The second one illustrates the use of multiple and different options on the two sides. 3.1 Call Agent Controlled T.38 Strict In this example, both sides are under strict T.38 Call Agent control. We assume the originating and terminating Call Agent communicate via the Session Initiation Protocol (SIP) [RFC3261] (also, see [SIPfax]): Andreasen Informational - Expires January 2005 12 MGCP Fax Package July 2004 ------------------------------------------------------------------ | #| GW-o | CA-o | CA-t | GW-t | |==|===============|===============|===============|===============| | 1| <-|CRCX | | | | 2| 200(sdp-o)|-> | | | | 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| | | | <- T.30 fax | | | | | | preamble | |11| | | <-|NTFY(t38 start)| |12| | | 200|-> | |13| | | MDCX(t38)|-> | |14| | | <-|200(sdp-t2) | |15| | <-|INVITE(sdp-t2) | | |16| <-|MDCX(sdp-t2) | | | |17| 200(sdp-o2)|-> | | | |18| | 200(sdp-o2)|-> | | |19| | | MDCX(sdp-o2)|-> | |20| | | <-|200 | |--|---------------|---------------|---------------|---------------| |21| | | | (fax ends) | |22| | | <-|NTFY(t38 stop) | |24| | | 200|-> | ------------------------------------------------------------------ Step 1: The Call Agent issues a CreateConnection command to the gateway instructing it to use PCMU media encoding and to use the strict Call Agent controlled T.38 procedure. Consequently, the Call Agent asks the gateway to notify it of the t38 event: CRCX 1000 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0 C: 1 L: a:PCMU, fxr/fx:t38 M: recvonly R: fxr/t38 X: 1 Step 2: The gateway acknowledges the command and includes SDP with codec information as well as RFC 3407 capability information: Andreasen Informational - Expires January 2005 13 MGCP Fax Package July 2004 200 1000 OK I:1 v=0 o=- 25678 753849 IN IP4 128.96.41.1 s=- c=IN IP4 128.96.41.1 t=0 0 m=audio 3456 RTP/AVP 0 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 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 PCMU media encoding and to use the strict Call Agent controlled T.38 procedure. Consequently, the Call Agent asks the gateway to notify it of the t38 event: CRCX 2000 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0 C: 2 L: a:PCMU, fxr/fx:t38 M: sendrecv R: fxr/t38 X: 20 v=0 o=- 25678 753849 IN IP4 128.96.41.1 s=- c=IN IP4 128.96.41.1 t=0 0 m=audio 3456 RTP/AVP 0 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 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 sends back a success response with its SDP which also includes capability information: Andreasen Informational - Expires January 2005 14 MGCP Fax Package July 2004 200 2000 OK I:2 v=0 o=- 25678 753849 IN IP4 128.96.41.2 s=- c=IN IP4 128.96.41.2 t=0 0 m=audio 1296 RTP/AVP 0 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 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: MDCX 1001 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0 C: 1 I: 1 M: sendrecv v=0 o=- 25678 753849 IN IP4 128.96.41.2 s=- c=IN IP4 128.96.41.2 t=0 0 m=audio 1296 RTP/AVP 0 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 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. Had there not been any support for T.38 in the RemoteConnectionDescriptor, then this command would still have succeeded, however there would be no special fax handling procedure (since strict mode could not be supported). Step 8: Andreasen Informational - Expires January 2005 15 MGCP Fax Package July 2004 The gateway acknowledges the command. At this point, a call is established using PCMU encoding, and if a fax call is detected, the Call Agent controlled T.38 procedure will be initiated. Step 9-11: A fax call now occurs. First, the T.30 CED tone (a.k.a. V.25 ANS) is sent which in this case is simply passed through the current PCMU encoding. 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 10, where the V.21 fax preamble is detected. The gateway was instructed to apply the Call Agent controlled T.38 procedure for fax calls, so the "t38(start)" event occurs and is notified to the Call Agent: NTFY 2500 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0 O: fxr/t38(start) X: 20 Step 12: The Call Agent acknowledges the Notify command: 200 2500 OK Step 13: The Call Agent then instructs the terminating gateway to change to using the "image/t38" MIME type instead: MDCX 2002 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0 C: 2 I: 2 L: a:image/t38 R: fxr/t38 X: 21 Step 14: The gateway changes to T.38, and sends back a success response with updated SDP: 200 2002 OK v=0 o=- 25678 753850 IN IP4 128.96.41.2 s=- c=IN IP4 128.96.41.2 t=0 0 m=image 1296 udptl t38 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 Andreasen Informational - Expires January 2005 16 MGCP Fax Package July 2004 Note, that since the gatewayÆs current RemoteConnectionDescriptor (as opposed to the LocalConnectionDescriptor returned here) does not list "image/t38" as a valid encoding method, the terminating gateway is still muting the media and is now waiting for an updated RemoteConnectionDescriptor with "image/t38". Step 15: The terminating Call Agent sends a re-INVITE to the originating Call Agent with the updated SDP. Step 16: The originating Call Agent then sends a ModifyConnection command to the originating gateway: MDCX 1003 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0 C: 1 I: 1 v=0 o=- 25678 753850 IN IP4 128.96.41.2 s=- c=IN IP4 128.96.41.2 t=0 0 m=image 1296 udptl t38 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 Step 17: The originating gateway changes to T.38 and sends back a success response with updated SDP: 200 1003 OK v=0 o=- 25678 753850 IN IP4 128.96.41.1 s=- c=IN IP4 128.96.41.1 t=0 0 m=image 3456 udptl t38 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 Step 18: 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). Andreasen Informational - Expires January 2005 17 MGCP Fax Package July 2004 Step 19: The terminating Call Agent sends a ModifyConnection with the updated SDP to the terminating gateway: MDCX 2003 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0 C: 2 I: 2 v=0 o=- 25678 753850 IN IP4 128.96.41.1 s=- c=IN IP4 128.96.41.1 t=0 0 m=image 3456 udptl t38 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 Step 20: The terminating gateway sends back a success response: 200 2003 OK Since the terminating gateway now has a RemoteConnectionDescriptor with "image/t38" as valid media, it can start exchanging T.38 with the originating gateway. Step 21, 22: When the fax ends, a "t38(stop)" event is generated, which is notified to the Call Agent: NTFY 2501 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0 O: t38(stop) X: 3 Step 23: The Call Agent acknowledges the Notify command: 200 2501 OK The fax call is now over. The Call Agent may now decide to change back to a voice codec, delete the connection, or something different. 3.2 Multiple and Different Options In this example, the originating gateway is instructed to use the gateway procedure whereas the terminating gateway is given a choice Andreasen Informational - Expires January 2005 18 MGCP Fax Package July 2004 between gateway procedure and strict t38 procedure. Furthermore, the originating fax machine is generating CNG tone. ------------------------------------------------------------------ | #| GW-o | CA-o | CA-t | GW-t | |==|===============|===============|===============|===============| | 1| <-|CRCX | | | | 2| 200(sdp-o)|-> | | | | 3| | INVITE(sdp-o)|-> | | | 4| | | CRCX(sdp-o)|-> | | 5| | | <-|200 (sdp-t) | | 6| | <-|200(sdp-t) | | | 7| <-|MDCX(sdp-t) | | | | 8| 200|-> | | | |--|---------------|---------------|---------------|---------------| | 9| CNG ->| | | | |10| | | |<- ANS/T.30 CED| |11| | | |<- T.30 fax p. | |12| | | <-|NTFY(t38 start)| |13| | | 200|-> | |14| | | MDCX(t38)|-> | |15| | | <-|200(sdp-t2) | |16| | <-|INVITE(sdp-t2) | | |17| <-|MDCX(sdp-t2) | | | |18| 200(sdp-o2)|-> | | | |19| | 200(sdp-o2)|-> | | |20| | | MDCX(sdp-o2)|-> | |21| | | <-|200 | |--|---------------|---------------|---------------|---------------| |22| | | | (fax ends) | |23| | | <-|NTFY(t38 stop) | |24| | | 200|-> | ------------------------------------------------------------------ Step 1: The Call Agent issues a CreateConnection command to the gateway instructing it to use PCMU media encoding and to use the gateway procedure. Consequently, the Call Agent asks the gateway to notify it of the gwfax event: CRCX 1000 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0 C: 1 L: a:PCMU, fxr/fx:gw M: recvonly R: fxr/gwfax X: 1 Step 2: The gateway acknowledges the command and includes SDP with codec information as well as capability information: Andreasen Informational - Expires January 2005 19 MGCP Fax Package July 2004 200 1000 OK I:1 v=0 o=- 25678 753849 IN IP4 128.96.41.1 s=- c=IN IP4 128.96.41.1 t=0 0 m=audio 3456 RTP/AVP 0 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 a=X-FaxScheme123 We assume the gateway supports some other fax scheme and it indicates this by including an attribute "FaxScheme123" 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 PCMU media encoding and to use either the gateway procedure or strict Call Agent controlled T.38 procedure. Consequently, the Call Agent asks the gateway to notify it of both the gwfax and t38 events: CRCX 2000 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0 C: 2 L: a:PCMU, fxr/fx:gw;t38 M: sendrecv R: fxr/t38, fxr/gwfax X: 20 v=0 o=- 25678 753849 IN IP4 128.96.41.1 s=- c=IN IP4 128.96.41.1 t=0 0 m=audio 3456 RTP/AVP 0 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 a=X-FaxScheme123 Step 5: The terminating gateway does not support any special gateway fax handling, however it does support T.38, and the RemoteConnectionDescriptor included indicates that the other side Andreasen Informational - Expires January 2005 20 MGCP Fax Package July 2004 supports T.38 as well, so the strict T.38 Call Agent controlled procedure requested can be honored. The terminating gateway sends back a success response with its SDP which also includes capability information: 200 2000 OK I:2 v=0 o=- 25678 753849 IN IP4 128.96.41.2 s=- c=IN IP4 128.96.41.2 t=0 0 m=audio 1296 RTP/AVP 0 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 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: MDCX 1001 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0 C: 1 I: 1 M: sendrecv v=0 o=- 25678 753849 IN IP4 128.96.41.2 s=- c=IN IP4 128.96.41.2 t=0 0 m=audio 1296 RTP/AVP 0 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 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, i.e. the gateway procedure. The SDP information returned however does not indicate support for the "FaxScheme123", and hence the originating gateway will not invoke any special fax handling procedure for this call. Andreasen Informational - Expires January 2005 21 MGCP Fax Package July 2004 Step 8: The gateway acknowledges the command. At this point, a call is established using PCMU encoding and if a fax call is detected, no special fax handling procedure will occur. Step 9-12: First, a CNG tone is generated by the originating fax thereby indicating a fax call. If the gateway was using either of the T.38 modes, or it had negotiated support for special gateway handling procedure with the other side, a "t38(start)" or "gwfax(start)" event would now have been generated and the switch to T.38 (or special gateway handling) could start. However, since the negotiation with the terminating gateway resulted in the originating gateway not doing anything special for fax, no such event is generated. Instead, the "nopfax(start)" event is now generated, but since the Call Agent has not requested this event, it is not detected and hence not reported to the Call Agent. Consequently, the CNG tone is simply passed through the current PCMU encoding without the (originating) Call Agent being aware of the fax call. Subsequently, the T.30 CED tone (a.k.a. V.25 ANS) occurs which in this case is also simply passed through the current PCMU encoding. 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 11, where the V.21 fax preamble is detected. The terminating gateway is using the Call Agent controlled T.38 procedure for fax calls, so the "t38(start)" event occurs and is notified to the Call Agent: NTFY 2500 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0 O: fxr/t38(start) X: 20 Step 13: The Call Agent acknowledges the Notify command: 200 2500 OK Step 14: The Call Agent then instructs the terminating gateway to change to using the "image/t38" MIME type instead: MDCX 2002 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0 C: 2 I: 2 L: a:image/t38 R: fxr/t38 X: 21 Andreasen Informational - Expires January 2005 22 MGCP Fax Package July 2004 Step 15: The gateway changes to T.38, and sends back a success response with updated SDP: 200 2002 OK v=0 o=- 25678 753850 IN IP4 128.96.41.2 s=- c=IN IP4 128.96.41.2 t=0 0 m=image 1296 udptl t38 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 Note, that since the terminating gatewayÆs last received RemoteConnectionDescriptor (as opposed to the LocalConnectionDescriptor returned here) did not list "image/t38" as a valid encoding method, the terminating gateway is still muting the media and is now waiting for an updated RemoteConnectionDescriptor with "image/t38". Step 16: The terminating Call Agent sends a re-INVITE to the originating Call Agent with the updated SDP. Step 17: The originating Call Agent then sends a ModifyConnection command to the originating gateway: MDCX 1003 ds/ds1-1/1@gw-o.whatever.net MGCP 1.0 C: 1 I: 1 v=0 o=- 25678 753850 IN IP4 128.96.41.2 s=- c=IN IP4 128.96.41.2 t=0 0 m=image 1296 udptl t38 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 Step 18: The originating gateway changes to T.38 and sends back a success response with updated SDP: Andreasen Informational - Expires January 2005 23 MGCP Fax Package July 2004 200 1003 OK v=0 o=- 25678 753850 IN IP4 128.96.41.1 s=- c=IN IP4 128.96.41.1 t=0 0 m=image 3456 udptl t38 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 Step 19: 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 20: The terminating Call Agent sends a ModifyConnection with the updated SDP to the terminating gateway: MDCX 2003 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0 C: 2 I: 2 v=0 o=- 25678 753850 IN IP4 128.96.41.1 s=- c=IN IP4 128.96.41.1 t=0 0 m=image 3456 udptl t38 a=sqn: 0 a=cdsc: 1 audio RTP/AVP 0 18 a=cdsc: 3 image udptl t38 Step 21: The terminating gateway sends back a success response: 200 2003 OK Since the terminating gateway now has a RemoteConnectionDescriptor with "image/t38" as valid media, it can start exchanging T.38 with the originating gateway. Step 22, 23: When the fax ends, a "t38(stop)" event is generated, which is notified to the Call Agent: NTFY 2501 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0 O: t38(stop) Andreasen Informational - Expires January 2005 24 MGCP Fax Package July 2004 X: 3 Step 24: The Call Agent acknowledges the Notify command: 200 2501 OK The fax call is now over. The Call Agent may now decide to change back to a voice codec, delete the connection, or something different. 4. Security Considerations The MGCP fax package itself is not known to introduce any new security concerns. However, implementers should note, that T.38 media is currently transported over UDP (UDPTL) or TCP in clear and without any integrity protection. If for example security services are in place to protect RTP media streams, these will thus not be in effect for the T.38 media stream. If such lack of security is a concern, the fax LocalConnectionOptions allowing T.38 in this package SHOULD NOT be used, i.e. the "off" (or a new secure extension) fax LocalConnectionOption should be used. 5. IANA Considerations The IANA is hereby requested to register the following MGCP package: Package Title Name Version ------------- ---- ------- Fax FXR 0 6. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3435] F. Andreasen, B. Foster, "Media Gateway Control Protocol (MGCP) Version 1.0", RFC 3435, January 2003. [T38] ITU-T Recommendation T.38, "Procedures for real-time Group 3 facsimile communication over IP networks", 03/2002. [RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple Capability Declaration", RFC 3407, October 2002. 7. Informative References [T30] ITU-T Recommendation T.30, "Procedures for document facsimile transmission in the general switched telephone network", 07/03. Andreasen Informational - Expires January 2005 25 MGCP Fax Package July 2004 [RFC3261] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J. Peterson, R. Sparks, M. Handley, E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [SIPfax] Mule, J., and J. Li, "SIP Support for Real-time Fax: Call Flow Examples and Best Current Practices", work in progress. 8. Acknowledgements Several people have contributed to the development of the MGCP fax package. In particular, the author would like to thank Gary Kelly, Rajesh Kumar, Dave Horwitz, Rob Thompson and the CableLabs PacketCable NCS focus team for their contributions. 9. Author's Address Flemming Andreasen Cisco Systems 499 Thornall Street, 8th Floor Edison, NJ 08837 Email: fandreas@cisco.com Andreasen Informational - Expires January 2005 26 MGCP Fax Package July 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the IETF's procedures with respect to rights in IETF Documents can be found in BCP 78 and BCP 79. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Andreasen Informational - Expires January 2005 27