Internet Engineering Task Force F. Andreasen Internet Draft Cisco Systems Document: draft-andreasen-mgcp-fax-00.txt February, 2002 Category: Informational MGCP Fax Package Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [1]. 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. Abstract This document defines an 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 RFC-2119 [2]. Andreasen Informational - Expires August 2002 1 MGCP Fax Package February 2002 Table of Contents 1. Fax Package Definition.........................................2 1.2 LocalConnectionOptions.......................................2 1.2.1 T.38 Mode (Strict or Loose)...............................4 1.2.3 Gateway Mode..............................................4 1.2.4 Off Mode..................................................5 1.2.5 Mode Operation............................................5 1.3 Events and Signals...........................................6 1.4 Connection Parameters........................................8 1.4 Additional Considerations....................................9 1.4.1 Media IP Address and Port.................................9 2. Call Flow Examples.............................................9 2.1 Call Agent Controlled T.38 Strict............................9 2.2 Multiple and Different Options..............................15 3. Security Considerations.......................................22 4. References....................................................22 5. Acknowledgements..............................................22 6. Author's Addresses............................................23 1. 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 1.2 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 to the Media Gateway. The fax parameter contains an ordered list of desired fax handling options. When the parameter is explicitly included, the gateway MUST be able to use at least one of the listed options for the command to succeed. The list may currently contain one or more of the following: * T.38 Strict Use T.38 [3] for fax relay and have the Call Agent control it. Assuming the procedure can be used, a switch to T.38 procedures will be initiated upon fax detection and a "t38(start)" event will be generated (see Section 1.3). This mode requires an indication of T.38 support from the remote side in order to be used. Andreasen Informational - Expires August 2002 2 MGCP Fax Package February 2002 * 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. * Off Do not invoke any special procedure for fax. * 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 option 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 1.3). The Call Agent SHOULD then refrain from issuing potentially conflicting commands to the gateway until the gateway ends it's special fax handling. A gateway that ends up not being able to invoke any special procedure for fax will generate a "nopfax(start)" event upon detection of fax. The set of possible values for the Fax LCO is extensible. The prefixes "x-", which indicates an optional extension, and "x+", which indicates a mandatory extension, are reserved for vendor specific use. In CreateConnection, the Fax LCO defaults to "gateway". In ModifyConnection, it defaults to its current value on the connection. If LocalConnectionOptions are either omitted or the fax parameter is not included, the previous fax parameter value for the connection will thus be retained, but without affecting the outcome of the command (consequently, the gateway may not apply any special procedure to fax - if the Call Agent wants to ensure that either a procedure is applied or the command fails, it must include the fax LCO parameter again). If multiple fax parameter values are provided, the gateway must choose one of the values. The fax parameter is encoded as the keyword "fx" (prefixed with the package name), 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". 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: Andreasen Informational - Expires August 2002 3 MGCP Fax Package February 2002 A: a:image/t38, fxr/fx:mypar, ... We now provide additional detail on the above defined fax modes. 1.2.1 T.38 Mode (Strict or Loose) When a gateway is instructed to operate in Call Agent controlled T.38 mode, the "m=" line in the SDP returned will not indicate T.38 (unless the gateway was also instructed to use T.38 for the media stream), however capability information for T.38 (if supported) using the SDP Simple Capability Negotiation extensions [7] 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 A gateway operating in Call Agent controlled T.38 mode that detects a fax 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 encoding method in the LocalConnectionOptions (receipt of a ModifyConnection command without LocalConnectionOptions but with a RemoteConnectionDescriptor containing an "m=" line with t38 would achieve the same). Per the normal codec negotiation procedures, if a RemoteConnectionDescriptor was included as well, it MUST include an "m=" line listing T.38 fax relay as an acceptable media format in order for the command to succeed. If a RemoteConnectionDescriptor was not included with the ModifyConnection command sent to a gateway under Call Agent T.38 control, it is possible (in fact likely), that the last received RemoteConnectionDescriptor did not include an "m=" line listing T.38 fax relay as an acceptable media format. In that case, the endpoint cannot send T.38 media. The endpoint will instead wait for an updated RemoteConnectionDescriptor with T.38 fax relay listed as an acceptable media format. If the fax call fails, e.g. due to a fax timeout, while waiting for the updated RemoteConnectionDescriptor with T.38, a "t38(failure)" event will be generated. 1.2.3 Gateway Mode Andreasen Informational - Expires August 2002 4 MGCP Fax Package February 2002 A gateway operating in Gateway controlled mode may initiate special fax handling upon detecting a fax call. The details of this special fax handling is outside the scope of this document. However, support for special fax handling MUST be negotiated with the other side by passing and recognizing relevant parameters via the SDP. 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. 1.2.4 Off Mode A gateway using the "off" mode 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 alternative voice codec as needed. 1.2.5 Mode Operation For each of the above modes, the RemoteConnectionDescriptor provides information on what procedure 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. 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 media format, e.g. T.38, is included in the "m=" line in the RemoteConnectionDescriptor, or * the relevant media format is included as a capability in the RemoteConnectionDescriptor. If the gateway cannot select any of the procedures in the Fax LocalConnectionOptions, the command fails. Note that "T.38 Loose", "gateway", and "off" by definition will not lead to failure. 3. If the Call Agent did not include any Fax LocalConnectionOptions or a RemoteConnectionDescriptor, the gateway MUST continue using whichever procedure it is currently using. Andreasen Informational - Expires August 2002 5 MGCP Fax Package February 2002 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. Note that this is not an issue for CreateConnection, since the default value can always be supported by definition. The following examples illustrate the use of the above rules. Per rule 1, a gateway that does not support T.38 fax relay will fail a command that only contains the fax option "t38 " whereas it will succeed a command that contains, "gw", "off" or no Fax LCO. A gateway supporting T.38 that receives a CreateConnection command with Fax handling 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 included either "t38-loose", "gw" or "off", the command would have succeeded and either 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 and with a RemoteConnectionDescriptor that has neither a T.38 capability nor a T.38 media stream, the command will succeed (note that rule 1 has no effect). However, per rule 2 and 4, there will be no T.38 procedure. Had the CA instead included a Fax 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 gateway 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 CNG and the terminating fax detects V.21 fax preamble before the switch to T.38 has been performed on the terminating side. 1.3 Events and Signals The following events are defined in support of the above: ------------------------------------------------------------------ | Symbol | Definition | R | S Duration | |---------|----------------------------|-----|---------------------| | gwfax | Gateway controlled fax | x | | | nopfax | No special fax handling | x | | | t38 | T.38 fax relay | x | | ------------------------------------------------------------------ Andreasen Informational - Expires August 2002 6 MGCP Fax Package February 2002 The definition of the individual events are as follows: Gateway Controlled Fax (gwfax): Gateway controlled fax handling. The gateway handled fax event is parameterized with one of the following: * start Gateway handled fax 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 handled fax ended normally. Media parameters for the connection are as before the gateway handled fax occurred. * failure The procedure ended abnormally. Media parameters are as before the gateway handled fax occurred. The "gwfax" event may be parameterized with additional parameters, however it is RECOMMENDED that one of the above parameters will be the first parameter supplied. The following example illustrates the encoding of the "gwfax" event: O: fxr/gwfax(start) O: fxr/gwfax(stop, foobar) No Special Fax Handling(nopfax): There is no special fax handling in place, however a fax call is now detected. This can happen either due to no special procedure being requested (including "off"), or negotiation leading to no special fax handling being possible. The event is parameterized with the following: * start No special fax handling is in place, however a fax call is now detected. The Call Agent may now have to issue commands in order to ensure a successful fax call. 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) T.38 fax relay(t38): Call Agent controlled T.38 fax relay. The Call Agent controlled T.38 fax relay event is parameterized with one of the following: Andreasen Informational - Expires August 2002 7 MGCP Fax Package February 2002 * start Call Agent controlled T.38 fax relay was initiated. The Call Agent SHOULD modify each side of the connection to start using T.38, unless they already do. * stop Call Agent controlled T.38 fax relay. The Call Agent may want to modify the media parameters for each side of the connection. * failure Call Agent controlled T.38 fax relay ended abnormally. The Call Agent may want to modify the media parameters for each side of the connection. The "t38" event may be parameterized with additional parameters, however it is RECOMMENDED that one of the above parameters will be the first parameter supplied. The following example illustrates the encoding of the "t38" event: O: fxr/t38(start) O: fxr/t38(foobar, stop) 1.4 Connection Parameters The connection parameters for the connection measuring packets and octets sent and received 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 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 August 2002 8 MGCP Fax Package February 2002 1.4 Additional Considerations 1.4.1 Media IP Address and Port 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 this will minimize Quality of Service interactions from the change. However, if an endpoint has a good reason, it MAY choose not to follow this recommendation. 2. 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. 2.1 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 SIP (see e.g. [6]): Andreasen Informational - Expires August 2002 9 MGCP Fax Package February 2002 ------------------------------------------------------------------ | #| 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 mode. 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 NCS 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 capability information: Andreasen Informational - Expires August 2002 10 MGCP Fax Package February 2002 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 mode. 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 NCS 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 mode requested can be honored. The terminating gateway sends back a success response with its SDP which also includes capability information: Andreasen Informational - Expires August 2002 11 MGCP Fax Package February 2002 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 gateway. 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 NCS 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. 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 strict Call Agent controlled T.38. 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. Step 8: The gateway acknowledges the command. At this point, a call is established is using PCMU encoding. Andreasen Informational - Expires August 2002 12 MGCP Fax Package February 2002 Step 9-11: First, the T.30 CED tone (aka. V.25 ANS) occurs 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 T.30 fax preamble is detected. Since the gateway is instructed to apply the Call Agent controlled T.38 procedure for fax calls, the "t38(start)" event occurs, which is notified to the Call Agent: NTFY 2500 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0 NCS 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 T.38 codec instead: MDCX 2002 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0 NCS 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 August 2002 13 MGCP Fax Package February 2002 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 NCS 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. Step 19: Andreasen Informational - Expires August 2002 14 MGCP Fax Package February 2002 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 NCS 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 NCS 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. 2.2 Multiple and Different Options In this example, the originating gateway is instructed to use the gateway mode whereas the terminating gateway is given a choice Andreasen Informational - Expires August 2002 15 MGCP Fax Package February 2002 between gateway mode and strict t38 mode. 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 mode. 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 NCS 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 August 2002 16 MGCP Fax Package February 2002 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 mode or strict Call Agent controlled T.38 mode. 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 NCS 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: Andreasen Informational - Expires August 2002 17 MGCP Fax Package February 2002 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 supports T.38 as well, so the strict T.38 Call Agent controlled mode 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 gateway. 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 NCS 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 mode. The SDP information returned however does not indicate support for the "FaxScheme123", and hence the originating gateway will not invoke any special fax handling for this call. Andreasen Informational - Expires August 2002 18 MGCP Fax Package February 2002 Step 8: The gateway acknowledges the command. At this point, a call is established is using PCMU encoding. 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 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, however 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 (aka. 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 T.30 fax preamble is detected. Since the terminating gateway is using the Call Agent controlled T.38 procedure for fax calls, the "t38(start)" event occurs, which is notified to the Call Agent: NTFY 2500 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0 NCS 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 T.38 codec instead: MDCX 2002 ds/ds1-1/2@gw-t.whatever.net MGCP 1.0 NCS 1.0 C: 2 I: 2 L: a:image/t38 Andreasen Informational - Expires August 2002 19 MGCP Fax Package February 2002 R: fxr/t38 X: 21 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 NCS 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: Andreasen Informational - Expires August 2002 20 MGCP Fax Package February 2002 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 19: The originating Call Agent sends a SIP 200 OK response with the updated SDP to the terminating Call Agent. 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 NCS 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: Andreasen Informational - Expires August 2002 21 MGCP Fax Package February 2002 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 NCS 1.0 O: t38(stop) 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. 3. 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 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. 4. References [1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 [3] ITU-T Recommendation T.38. [4] ITU-T Recommendation T.38 Annex D, "SIP/SDP Call Establishment Procedures". [5] ITU-T T.38, Amendment 1, 4/99. [6] SIP T.38 Call Flow Examples And Best Current Practices, , work in progress. [7] SDP Simple Capability Negotiation, , work in progress. 5. Acknowledgements Andreasen Informational - Expires August 2002 22 MGCP Fax Package February 2002 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 and the PacketCable NCS focus team for their contributions. 6. Author's Addresses Flemming Andreasen Cisco Systems 499 Thornall Street, 8th Floor Edison, NJ 08837 Email: fandreas@cisco.com Andreasen Informational - Expires August 2002 23 MGCP Fax Package February 2002 Full Copyright Statement "Copyright (C) The Internet Society (2002). All Rights Reserved. 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