One document matched: draft-andreasen-mgcp-fax-03.txt
Differences from draft-andreasen-mgcp-fax-02.txt
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
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The list of current Internet-Drafts can be accessed at
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The list of Internet-Draft Shadow Directories can be accessed at
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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].
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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
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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.
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* 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".
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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
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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
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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.
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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:
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------------------------------------------------------------------
| 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)
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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
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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, ...
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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]):
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------------------------------------------------------------------
| #| 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:
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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:
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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:
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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
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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).
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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
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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:
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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
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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.
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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
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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:
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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)
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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.
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[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
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