One document matched: draft-andreasen-mgcp-fax-01.txt
Differences from draft-andreasen-mgcp-fax-00.txt
Internet Engineering Task Force F. Andreasen
Internet Draft Cisco Systems
Document: draft-andreasen-mgcp-fax-01.txt January, 2003
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-
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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 BCP 14, RFC-2119
[2].
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Table of Contents
1. Fax Package Definition.............................................3
1.2 LocalConnectionOptions..........................................3
1.2.1 T.38 Mode (Strict or Loose)..................................4
1.2.2 Gateway Mode.................................................5
1.2.3 Off Mode.....................................................5
1.2.4 Mode Operation...............................................6
1.3 Events and Signals..............................................7
1.4 Connection Parameters...........................................9
1.5 Additional Considerations......................................10
1.5.1 Media IP Address and Port...................................10
2. Call Flow Examples................................................10
2.1 Call Agent Controlled T.38 Strict..............................10
2.2 Multiple and Different Options.................................16
3. Security Considerations...........................................23
4. References........................................................23
5. Acknowledgements..................................................23
6. Author's Address..................................................24
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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.
* 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
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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. Please refer to
Section 1.2.5 for further details.
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:
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 Declaration 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).
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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(stop)" or a
"t38(failure)" event will be generated.
1.2.2 Gateway Mode
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, in
order to use special fax handling, support for it 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.
1.2.3 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.
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1.2.4 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 (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 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 (see [7])
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 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.
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 1.2). Note that this is not
an issue for CreateConnection, since the default value can always
be supported by definition.
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.
T.38), the most recently received RemoteConnectionDescriptor MUST
include a corresponding media line.
The following examples illustrate the use of the above rules.
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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" may or may
not succeed (depending on the RemoteConnectionDescriptor).
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 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 T.38 media stream, 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 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 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 | |
------------------------------------------------------------------
The definition of the individual events are as follows:
Gateway Controlled Fax (gwfax):
Gateway controlled fax handling. The gateway controlled fax event
is parameterized with one of the following:
* start Gateway controlled 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.
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* stop Gateway controlled fax 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 occurred.
* failure The 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 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 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:
* 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 ended and
the gateway did not detect any errors. Note that this does not
necessarily imply a successfully transmitted fax. It merely
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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.
* failure Call Agent controlled T.38 fax relay ended
abnormally. Some kind of problem in the Call Agent 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.
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, ...
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1.5 Additional Considerations
1.5.1 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]):
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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 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
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:
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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 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
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:
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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 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
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.
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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
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
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
Note, that since the gatewayÆs current RemoteConnectionDescriptor
(as opposed to the LocalConnectionDescriptor returned here) does not
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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.
Step 19:
The terminating Call Agent sends a ModifyConnection with the updated
SDP to the terminating gateway:
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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.
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
between gateway mode and strict t38 mode. Furthermore, the
originating fax machine is generating CNG tone.
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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| 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
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 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
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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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
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.
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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
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
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.
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)
X: 3
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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] Mule, J., and J. Li, "SIP Support for Real-time Fax: Call Flow
Examples and Best Current Practices", work in progress.
[7] F. Andreasen, "Session Description Protocol (SDP) Simple
Capability Declaration", RFC 3407, October 2002.
5. 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 PacketCable NCS
focus team for their contributions.
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6. Author's Address
Flemming Andreasen
Cisco Systems
499 Thornall Street, 8th Floor
Edison, NJ 08837
Email: fandreas@cisco.com
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