Internet Engineering Task Force Jean-Francois Mule
Internet Draft
Document: <draft-mule-sip-t38callflows-02.txt> Jieying Li
Category: Informational Clarent Corp
Expires: April 2002 October 2001
SIP Real-time Fax Call Flow Examples
And
Best Current Practices
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.
1. Abstract
The Session Initiation Protocol (SIP) allows the establishment of
real-time Internet fax communications. Real-time facsimile
communications over IP may follow 2 modes of operation: T.38 fax
relay as defined by the ITU-T T.38 recommendation or fax pass-
through.
This document clarifies the options available to Internet telephony
gateway vendors to handle real-time fax calls using SIP. While our
primary focus is to address the more reliable real-time T.38 Group 3
fax mode, we briefly cover the fax pass-through mode to enable
fallback operations and super G3 fax communications using SIP. We
also give examples of SIP call flows for real-time Internet fax
gateways or SIP proxy redirect servers. Elements in these call
flows include SIP User Agents, SIP Proxy Servers, and Gateways to
the PSTN (Public Switch Telephone Network).
This document introduces best current practices for SIP T.38 fax and
SIP fax pass-through sessions. A session starts with audio
capabilities, and, upon fax tone detection, T.38 fax capabilities
are negotiated; upon successful negotiation, the session continues
with fax capabilities and the media termination hosts exchange T.38
Internet fax packets. The T.38 fax call scenarios include various
aspects of the call sequence: the detection of fax transmission, the
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SIP T.38 Call Flows October 2001
usage of the T.38 session description attributes, the optional
fallback into fax pass-through mode and the session termination. The
fax pass-through call scenarios involve some specific SDP media
attributes to enable proper fax transmission.
Fax transmission can be detected by the receiving side, the emitting
side or both (in the latter case, a 'glare' effect may appear).
This document only covers the case when the fax transmission is
detected by the receiving side: it is the most common practice and
the other cases do not represent any particular challenges and are
therefore left for future discussions). Call flow diagrams and
message details are shown. A list of IANA defined SDP attribute
names for T.38 is summarized in section 7.
Table of Contents
Status of this Memo...................................................1
1. Abstract...........................................................1
2. Conventions used in this document..................................3
3. Overview...........................................................3
3.1. General Assumptions.............................................3
3.2 Legend for Message Flows..........................................4
3.3 Changes to previous drafts........................................4
4 Handling of real-time fax communication using SIP...................5
4.1 Internet telephony gateways and fax detection.....................5
4.2 Internet telephony gateways and T.38 fax media connections........5
4.3 SIP session and fax termination...................................6
4.4 Internet telephony gateways and fax pass-through mode.............6
4.5 Responses to OPTIONS request......................................7
4.6 Beyond Internet telephony gateways................................7
5 Successful SIP T.38 fax Call Scenarios..............................7
5.1 Internet fax device û fax only support............................7
5.1.1 Sequence Diagram................................................7
5.1.2 Message Details.................................................8
5.2 SIP T.38 fax call û fax stream replaces voice stream.............13
5.2.1 Sequence Diagram...............................................14
5.2.2 Message Details................................................15
5.3 SIP T.38 fax call û fax stream added to voice stream.............25
5.3.1 Sequence Diagram...............................................26
5.3.2 Message Details................................................27
5.4 SIP T.38 fax call û fax stream forced to replace voice stream....37
5.4.1 Sequence Diagram...............................................37
5.4.2 Message Details................................................38
6. Unsuccessful Scenarios............................................41
6.1. Unsuccessful T.38 fax scenario û no support for T.38............41
6.1.1. Sequence Diagram..............................................41
6.1.2. Message Details...............................................42
6.2. Unsuccessful T.38 fax scenario û 606 response and G.711 fallback44
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6.2.1. Sequence Diagram..............................................44
6.2.2. Message Details...............................................45
7. SDP Attribute Table for T.38 sessions.............................50
8. Considerations....................................................51
8.1. Level of requirements û solved, see section 4.2:................51
8.2. Negotiation of UDP ports for T.38 transmission..................51
8.3. Adding a fax stream to SIP session vs. Replacing audio stream...51
8.4. Adding text to make use of SIP OPTIONS: in progress in draft-02.51
8.5. Use of IP addresses in SDP connection information...............52
8.6. Unsuccessful scenario: 606 or 488...............................52
9. Security Considerations...........................................52
10. References.......................................................52
11. Acknowledgments..................................................53
12. Author's Addresses...............................................53
Full Copyright Statement.............................................53
2. 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].
3. Overview
The Session Initiation Protocol (SIP) is defined by RFC2543 [3] and
T.38 is the ITU-T Recommendation defining the procedures for real-
time Group 3 facsimile communications over IP networks [4]. This
document deals with the handling of real-time fax communications
using the SIP protocol for T.38 fax and fax pass-through modes.
For T.38, this document deals primarily with one transport protocol
for the media: T.38 over UDP/UDPTL; T.38 fax packet transport over
TCP using SIP session establishment can easily be extrapolated.
These T.38 call flows were developed in the design of carrier-class
SIP Telephony products supporting voice and real-time fax traffic.
It is the hope of the authors that this document will be useful for
the SIP community, SIP implementors of T.38 fax products, designers.
Call flows shown in this document complement the main SIP call flow
Internet-Draft [5]. These call flows are based on the current
version 2.0 of SIP in RFC2543[3], the ITU-T T.38 Amendment 2 [6] and
an ITU-T SG8 liaison to IANA. Note: IANA registration of the T.38
SDP attributes is currently a work in progress.
3.1. General Assumptions
Internet telephony gateways with T.38 support MAY pass various fax
tones (T.30 CNG, CED, etc.) or flag sequence (like V.21 Preamble) in
the RTP audio streams before those tones or flag sequence are
detected. Once detected by the DSP resources, T.38 fax-capable
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SIP T.38 Call Flows October 2001
devices switch from audio to fax mode and initiate a T.38 fax packet
transmission.
These best current practices MAY apply to "Network Gateway",
"Enterprise Gateway" and Internet-aware fax devices or terminals.
3.2 Legend for Message Flows
The legend defined [5] also applies in this document.
Dashed lines (---) represent control messages that are mandatory to
the call scenario. These control messages can be SIP or PSTN
signaling. Double dashed lines (===) represent media paths between
network elements. Messages with parenthesis around name represent
optional control messages. Messages are identified in the Figures
as F1, F2, etc. This references the message details in the table
that follows the Figure. Comments in the message details are shown
in the following form: /* Comments. */
3.3 Changes to previous drafts
o Changes to draft-01
The changes between the current draft and draft-01 are:
- Major updates:
+ additions based on SIPit fax testing event 8/2001: added
text regarding the negotiation of T38MaxBitRate in 4.2,
+ added more general text for the handling of real-time
fax pass-through,
+ added of echo cancellation & silence suppression SDP
attributes and usage recommendations in 6.2,
+ correction of the session version number in SDP when
issuing re-INVITE with updated SDP information as per
draft-ietf-sip-rfc2543bis-05,
+ started a section on the use of the OPTIONS method to
determine the support of T.38 capabilities (work-in-
progress),
- Minor editorial updates (added tag fields in To and From header
lines as well as Contact header lines as per draft-ietf-sip-
rfc2543bis-05, added proper reference to T.38 Amendment 2 pre-
published version, exact case matching for T38MaxBitRate, etc.)
o Changes to draft-00
The major changes between draft-01 and draft-00 are listed below:
- Major updates:
+ new text in fax handling section 4.2 based on sip list:
clarified text for fax detection, capability negotiation
and termination
+ additions of new successful call flows in section 5
+ additions of unsuccessful call flows in section 6
- Added Record-Route header, modified Cseq numbers,
- Added T38FaxMaxDatagram attribute in SDP and corrected
the sample value for T38FaxMaxBuffer,
- Minor editorial changes (e.g. removed references in Abstract).
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4 Handling of real-time fax communication using SIP
The handling of real-time fax sessions using SIP involves the
initial session establishment, the modification or negotiation of
session capabilities upon fax detection, the optional fallback into
fax pass-through mode and the session termination.
Some variants of these best current practices MAY apply depending on
the nature or the configuration of Internet telephony gateways. Two
distinct cases are considered in this document:
o The Internet telephony gateway only supports T.38 real-time
fax communications (by design or by configuration). In this case,
the Internet fax gateway SHOULD initiate the SIP session with T.38
SDP capabilities (this is typically the case of Internet fax
terminals, also called Internet-aware fax devices or the case of
gateways statically configured to support T.38 fax calls only);
o The Internet telephony gateway supports voice and real-time
fax communications. In this case, the Internet telephony gateway
initiates the SIP session with audio capabilities, and, upon fax
detection, the switchover to T.38 fax capabilities is triggered.
The fax media connection may replace or be added to the audio
connection depending on the target applications.
4.1 Internet telephony gateways and fax detection
Two possible ways exist for an Internet telephony gateway to detect
a facsimile communication:
1. On the emitting gateway (the one sending the facsimile document),
the T.30 calling tone (CNG) is detected. The CNG tone is however an
optional signal [7];
2. On the receiving gateway, the V.21 Preamble flag sequence is
detected. The Preamble is always present and follows the called
station identification tone (CED) when CED is present.
Note that even though the CED tone may be detected on the receiving
gateway, since the fax CED tone is similar to V.25 ANS modem tones,
it SHOULD NOT be used to trigger the fax switchover.
In conclusion, several conditions can trigger fax detection.
Internet telephony gateways supporting T.38 real-time facsimile MUST
support the detection of fax transmission on the V.21 Preamble flag
sequence; Internet telephony gateways MAY support the detection of
fax on CNG tone. The authors recommend that a facsimile
transmission SHOULD be detected on the receiving Internet telephony
gateway.
4.2 Internet telephony gateways and T.38 fax media connections
Upon successful fax detection, the description of the media
connection(s) is modified using the SDP protocol [8] as described in
SIP (see sections in chapter 5).
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Several cases should be considered:
- In the case of Internet fax devices (supporting T.38 fax only
communications), the SIP session is established with SDP description
containing the T.38 media attributes; refer to section 5.1 for a
sample call flow for Internet fax devices.
- In the case of Internet telephony gateways, the SIP/SDP session is
modified and the T.38 fax media connection SHOULD replace the
existing audio media connection. The SIP UAC triggering the
modification of the session description MUST indicate all of its
T.38 media attributes in SDP. This is the recommended behavior for
gateways; refer to section 5.2 for a sample call flow illustrating
this scenario.
Some implementations MAY choose to add the fax media connection to
the existing audio connection and in that case, they SHOULD mute the
audio connection while fax is transmitted. Refer to section 5.3 for
a sample call flow illustrating this optional scenario.
Note:
Among the SDP T.38 media attributes being exchanged after fax
detection, implementors should particularly pay attention to
T38MaxBitRate, the maximum T.38 bit rate for the media connection.
It is recommended to implement the following rule when responding to
a maximum bit rate proposal. If the remote peer's SDP description
contains a maximum fax bit rate of X and the local host is set to a
maximum bit rate of Y (either by fax protocol detection or
configuration), the local host MUST propose a maximum fax rate of
min (X,Y) in the SIP response to the re-INVITE in order to
facilitate the negotiation. This note is exemplified in section
5.2.1. in messages F11-F14. Failures to implement this rule MAY
imply call teardown in some cases.
4.3 SIP session and fax termination
Upon completion of the fax transmission, any audio connection
successfully established prior to fax detection SHOULD be restored
or the session MUST be terminated.
4.4 Internet telephony gateways and fax pass-through mode
This section deals real-time facsimile communications over IP using
fax pass-through instead of T.38 fax relay. In this mode, the
facsimile communication is handled as a PCM audio call (PCMA/PCMU as
specified in ITU-T recommendation G.711).
The fax pass-through mode is important to prevent call failures, for
example in cases when one of the SIP communication peers does not
support T.38. For Internet telephony gateways with support for
PCM/G.711 audio but no support for T.38 real-time fax, it is
recommended to switch the session to fax pass-through mode.
Internet telephony gateways SHOULD handle the fall back mode to fax
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pass-through by recognizing the SDP T.38 connection and proposing to
switch to a new audio connection. The new audio connection SHOULD
have the following characteristics: at a minimum, specify PCM G.711
codec, silence suppression OFF, echo cancellation ON. The telephony
gateway that originated the failed T.38 re-INVITE SHOULD initiate
the subsequent re-INVITE to fax pass-through mode.
Refer to section 6.2 for a sample call flow illustrating this
scenario.
4.5 Responses to SIP OPTIONS request
Suggestion currently logged in section 8.4, just a placeholder for
now; <<work in progress >>
4.6 Beyond Internet telephony gateways
This document may also serve to vendors of SIP redirect servers or
other SIP UA types like SIP phones. Such SIP entities may recognize
the fax nature of the sessions and redirect the flows to the proper
real-time fax-capable devices or to other fax service providers.
5 Successful SIP T.38 fax Call Scenarios
5.1 Internet fax device û fax only support
This section represents a SIP call flow for a T.38 fax session
between 2 Internet fax terminals. It is also applicable to Internet
telephony gateways when telephony ports are statically configured to
support fax only (case of analog IP telephony gateways where a
specific port is connected to a fax terminal).
A session starts with fax capabilities (t.38/udptl). The mechanism
for supporting T.38 in SIP & SDP is detailed in T.38 Annex D [9], a
temporary document that proposes an amendment to T.38 describing SIP
call establishment procedures.
5.1.1 Sequence Diagram
Internet Fax UA Proxy IFTGW UA
Fax | | |
------->| F1 INVITE | |
emitted |------------------->| |
| | F2 INVITE |
| |------------------->|
| F3 100 Trying | |
|<-------------------| F4 100 Trying |
| |<-------------------|
| | |
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| | F5 180 Ringing |
| F6 180 Ringing |<-------------------|
|<-------------------| |
| | F7 200 OK |
| F8 200 OK |<-------------------|
|<-------------------| |
| F9 ACK | |
|------------------->| F10 ACK |
| |------------------->|
| |
| T.38/UDPTL Fax Flow Established |
|<=======================================>|
End of | |
fax | | |
detected| | |
------->| | |
| F11 BYE | |
|------------------->| |
| | F12 BYE |
| |------------------->|
| | F13 200 OK |
| |<-------------------|
| F14 200 OK | |
|<-------------------| |
5.1.2 Message Details
F1 INVITE I.FAX UA -> PROXY
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP ifax.here.com:5060
From: sip:+1-303-555-1111@ifax.here.com;user=phone;tag=ab111
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ifax.here.com
CSeq: 17 INVITE
Contact: <sip:+1-303-555-1111@ifax.here.com>
Content-Type: application/sdp
Content-Length: 320
v=0
o=ifaxgw1 2890846527 2890846527 IN IP4 ifax.here.com
s=Session SDP
c=IN IP4 ifaxmg.here.com
t=0 0
m=image 15002 udptl t38
a=T38FaxVersion:0
a=T38MaxBitRate:14400
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
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a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPFEC
a=T38FaxUdpEC:t38UDPRedundancy
F2 INVITE PROXY -> IFTGW UA
INVITE sip:+1-650-555-2222@iftgw.there.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ifax.here.com:5060
Record-Route: <sip:+1-650-555-2222@iftgw.there.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ifax.here.com;user=phone;tag=ab111
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ifax.here.com
CSeq: 17 INVITE
Contact: <sip:+1-303-555-1111@ifax.here.com>
Content-Type: application/sdp
Content-Length: 320
v=0
o=ifaxgw1 2890846527 2890846527 IN IP4 ifax.here.com
s=Session SDP
c=IN IP4 ifaxmg.here.com
t=0 0
m=image 15002 udptl t38
a=T38FaxVersion:0
a=T38MaxBitRate:14400
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPFEC
a=T38FaxUdpEC:t38UDPRedundancy
F3 (100 Trying) PROXY -> I.FAX UA
SIP/2.0 100 Trying
Via: SIP/2.0/UDP ifax.here.com:5060
From: sip:+1-303-555-1111@ifax.here.com;user=phone;tag=ab111
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ifax.here.com
CSeq: 17 INVITE
Content-Length: 0
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F4 (100 Trying) IFTGW UA -> PROXY
SIP/2.0 100 Trying
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ifax.here.com:5060
From: sip:+1-303-555-1111@ifax.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ifax.here.com
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Length: 0
F5 180 Ringing IFTGW UA -> PROXY
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ifax.here.com:5060
From: sip:+1-303-555-1111@ifax.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ifax.here.com
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Length: 0
F6 180 Ringing PROXY -> I.FAX UA
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP ifax.here.com:5060
From: sip:+1-303-555-1111@ifax.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ifax.here.com
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Length: 0
F7 200 OK IFTGW UA -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ifax.here.com:5060
From: sip:+1-303-555-1111@ifax.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ifax.here.com
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 320
v=0
o=faxgw1 2890844527 2890844527 IN IP4 iftgw.there.com
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s=Session SDP
c=IN IP4 iftmg.there.com
t=0 0
m=image 49172 udptl t38
a=T38FaxVersion:0
a=T38MaxBitRate:14400
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPRedundancy
F8 200 OK PROXY -> I.FAX UA
SIP/2.0 200 OK
Via: SIP/2.0/UDP ifax.here.com:5060
Record-Route: <sip:+1-650-555-2222@iftgw.there.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ifax.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ifax.here.com
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 320
v=0
o=faxgw1 2890844527 2890844527 IN IP4 iftgw.there.com
s=Session SDP
c=IN IP4 iftmg.there.com
t=0 0
m=image 49172 udptl t38
a=T38FaxVersion:0
a=T38MaxBitRate:14400
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPRedundancy
F9 ACK I.FAX UA -> PROXY
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP ifax.here.com:5060
Route: <sip:+1-650-555-2222@iftgw.there.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ifax.here.com;user=phone;tag=ab11
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To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ifax.here.com
CSeq: 17 ACK
Content-Length: 0
F10 ACK PROXY -> IFTGW UA
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ifax.here.com:5060
Record-Route: <sip:+1-650-555-2222@iftgw.there.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ifax.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ifax.here.com
CSeq: 17 ACK
Content-Length: 0
/* T.38 fax streams are established. */
F11 BYE I.FAX UA -> PROXY
BYE sip:+1-650-555-2222@obelix.wcom.com SIP/2.0
Via: SIP/2.0/UDP ifax.here.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ifax.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ifax.here.com
CSeq: 18 BYE
Content-Length: 0
F12 BYE PROXY -> IFTGW UA
BYE sip:+1-650-555-2222@obelix.wcom.com SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d009.1
Via: SIP/2.0/UDP ifax.here.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ifax.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ifax.here.com
CSeq: 18 BYE
Content-Length: 0
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F13 200 OK IFTGW UA -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ifax.here.com:5060
From: sip:+1-303-555-1111@ifax.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde34
Call-ID: 1717@ifax.here.com
CSeq: 18 BYE
Content-Type: application/sdp
Content-Length: 0
F14 200 OK PROXY -> I.FAX UA
SIP/2.0 200 OK
Via: SIP/2.0/UDP ifax.here.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ifax.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde34
Call-ID: 1717@ifax.here.com
CSeq: 18 BYE
Content-Type: application/sdp
Content-Length: 0
5.2 SIP T.38 fax call û fax stream replaces voice stream
This section represents the SIP call flow for a T.38 fax session
between 2 Internet telephony gateways (fax detection on terminating
side).
A session starts with audio capabilities, and then the session is
modified to switch to t38 fax mode (t.38/udptl).
Scenario:
1. A SIP INVITE is sent to the called party requesting a voice
connection per RFC2543; our scenario involves 1 SIP proxy. An audio
connection is established,
2. Upon detection of Preamble by the terminating gateway, a SIP re-
INVITE request is sent to the emitting gateway to modify the
parameters of the session to allow a T.38 fax media connection.
This INVITE request contains new SDP information detailing T.38
capabilities,
3. Upon successful acknowledgments, T.38 IFP fax packets are
sent/received on UDP ports, which may be the same UDP ports used for
audio RTP,
4. Once the fax transmission is terminated, audio capabilities are
ôrestoredö or the call is terminated.
Li/Mule 13
SIP T.38 Call Flows October 2001
5.2.1 Sequence Diagram
In our example, we illustrate the fact that Internet telephony
gateways may use multiple network interfaces for signaling and/or
media streaming or one network interface with multiple IP addresses.
Typically, our example shows:
- one interface for SIP signaling
(ingress gateway = ift.here.com, egress gateway = iftgw.there.com)
- one or multiple interface(s) for media transport:
(ingress gateway = iftmg.here.com, egress gateway = iftmg.there.com)
A proxy is acting as a pure SIP signaling proxy (obelix.wcom.com).
IFT UA Proxy IFTGW UA
| | |
| F1 INVITE | |
|------------------->| |
| | F2 INVITE |
| |------------------->|
| F3 100 Trying | |
|<-------------------| F4 100 Trying |
| |<-------------------|
| | |
| | F5 180 Ringing |
| F6 180 Ringing |<-------------------|
|<-------------------| |
| | F7 200 OK |
| F8 200 OK |<-------------------|
|<-------------------| |
| | |
| F9 ACK | |
|------------------->| F10 ACK |
| ------------------->|
| |
| Both Way RTP Media Established |
Fax |<=======================================>|
------->| |
emitted | | |
| | | Preamble
| | |<---------
| | F11 INVITE | detected
| |<-------------------|
| F12 INVITE | |
|<-------------------| |
| | |
| F13 200 OK | |
|------------------->| |
| | F14 200 OK |
| |------------------->|
| | |
| | F15 ACK |
| |<-------------------|
| F16 ACK | |
|<-------------------| |
Li/Mule 14
SIP T.38 Call Flows October 2001
| |
| T.38/UDPT Fax Flow Established |
|<=======================================>|
End of | |
fax | | |
detected| | |
------->| | |
| | | End of fax
| | |<----------
| | | detected
| | F17 INVITE |
| |<-------------------|
| F18 INVITE | |
|<-------------------| |
| | |
| F19 200 OK | |
|------------------->| |
| | F20 200 OK |
| |------------------->|
| | |
| | F21 ACK |
| |<-------------------|
| F22 ACK | |
|<-------------------| |
| Both Way RTP Media Re-Established |
|<=======================================>|
| F23 BYE | |
|------------------->| |
| | F24 BYE |
| |------------------->|
| | |
| | F25 200 OK |
| |<-------------------|
| F26 200 OK | |
|<-------------------| |
5.2.2 Message Details
F1 INVITE IFT UA -> PROXY
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 146
Li/Mule 15
SIP T.38 Call Flows October 2001
v=0
o=IFAXTERMINAL01 2890844527 2890844527 IN IP4 ift.here.com
s=Session SDP
c=IN IP4 iftmg.here.com
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F2 INVITE PROXY -> IFTGW UA
INVITE sip:+1-650-555-2222@iftgw.there.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ift.here.com:5060
Record-Route: <sip:+1-650-555-2222@iftgw.there.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 146
v=0
o=IFAXTERMINAL01 2890844527 2890844527 IN IP4 ift.here.com
s=Session SDP
c=IN IP4 iftmg.here.com
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F3 (100 Trying) PROXY -> IFT UA
SIP/2.0 100 Trying
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Content-Length: 0
F4 100 Trying IFTGW UA -> PROXY
SIP/2.0 100 Trying
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
Li/Mule 16
SIP T.38 Call Flows October 2001
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Length: 0
F5 180 Ringing IFTGW UA -> PROXY
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Length: 0
F6 180 Ringing PROXY -> IFT UA
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Length: 0
F7 200 OK IFTGW UA -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 150
v=0
o=IFAXTERMINAL01 2890844527 171089 IN IP4 iftgw.there.com
s=Session SDP
c=IN IP4 iftmg.there.com
t=0 0
m=audio 12322 RTP/AVP 0
a=rtpmap:0 PCMU/8000
Li/Mule 17
SIP T.38 Call Flows October 2001
F8 200 OK PROXY -> IFT UA
SIP/2.0 200 OK
Via: SIP/2.0/UDP ift.here.com:5060
Record-Route: <sip:+1-650-555-2222@iftgw.there.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 150
v=0
o=IFAXTERMINAL01 2890844527 171089 IN IP4 iftgw.there.com
s=Session SDP
c=IN IP4 iftmg.there.com
t=0 0
m=audio 12322 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F9 ACK IFT UA -> PROXY
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP ift.here.com:5060
Route: <sip:+1-650-555-2222@iftgw.there.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 17 ACK
Content-Length: 0
F10 ACK PROXY -> IFTGW UA
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ift.here.com:5060
Record-Route: <sip:+1-650-555-2222@iftgw.there.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 17 ACK
Content-Length: 0
Li/Mule 18
SIP T.38 Call Flows October 2001
/* RTP streams are established. The CNG fax tone is sent in-band if
it is present. The receiving side IFT gateway DSP detects the
Preamble sequence. A new UDP port is open on IFTGW for T.38 IFP
packets and the IFTGW signals the switch over to fax mode by send a
re-INVITE with the new UDP port in the SDP.
Note: the IFTGW has a T38MaxBitRate configured to 9600 */
F11 INVITE IFTGW UA -> PROXY
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP iftgw.there.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 320
v=0
o=faxgw1 2890844527 171090 IN IP4 iftgw.there.com
s=Session SDP
c=IN IP4 iftmg.there.com
t=0 0
m=image 49172 udptl t38
a=T38FaxVersion:0
a=T38MaxBitRate:9600
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPFEC
a=T38FaxUdpEC:t38UDPRedundancy
F12 INVITE PROXY -> IFT UA
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d008.1
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Li/Mule 19
SIP T.38 Call Flows October 2001
Content-Length: 320
v=0
o=faxgw1 2890844527 171090 IN IP4 iftgw.there.com
s=Session SDP
c=IN IP4 iftmg.there.com
t=0 0
m=image 49172 udptl t38
a=T38FaxVersion:0
a=T38MaxBitRate:9600
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPFEC
a=T38FaxUdpEC:t38UDPRedundancy
/* Note: the IFT has a T38MaxBitRate configured to 14400 but since
it received a T38MaxBitRate proposal of 9600, it replies with
T38MaxBitRate = min (9600,14400) */
F13 200 OK IFT UA -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d008.1
Via: SIP/2.0/UDP iftgw.there.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 320
v=0
o=faxgw1 2890846527 2890846528 IN IP4 ift.here.com
s=Session SDP
c=IN IP4 iftmg.here.com
t=0 0
m=image 15002 udptl t38
a=T38FaxVersion:0
a=T38MaxBitRate:9600
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPRedundancy
Li/Mule 20
SIP T.38 Call Flows October 2001
F14 200 OK PROXY -> IFT UA
SIP/2.0 200 OK
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 320
v=0
o=faxgw1 2890846527 2890846528 IN IP4 ift.here.com
s=Session SDP
c=IN IP4 iftmg.here.com
t=0 0
m=image 15002 udptl t38
a=T38FaxVersion:0
a=T38MaxBitRate:9600
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPRedundancy
F15 ACK IFTGW UA -> PROXY
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP iftgw.there.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 ACK
Content-Length: 0
F16 ACK PROXY -> IFT UA
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d008.1
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Li/Mule 21
SIP T.38 Call Flows October 2001
Call-ID: 1717@ift.here.com
CSeq: 18 ACK
Content-Length: 0
/* T.38 fax transmission established both ways */
/* Then, the end of the fax transmission is detected on ingress side
and sent to the egress side (IFTGW). IFTGW initiates the switch
back to voice communication */
F17 INVITE IFTGW UA -> PROXY
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP iftgw.there.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 181
v=0
o=faxgw1 2890844527 171091 IN IP4 iftgw.there.com
s=Session SDP
c=IN IP4 iftmg.there.com
t=0 0
m=audio 12322 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F18 INVITE PROXY -> IFT UA
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d009.1
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 181
v=0
o=faxgw1 2890844527 171091 IN IP4 iftgw.there.com
s=Session SDP
Li/Mule 22
SIP T.38 Call Flows October 2001
c=IN IP4 iftmg.there.com
t=0 0
m=audio 12322 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F19 200 OK IFT UA -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d008.1
Via: SIP/2.0/UDP iftgw.there.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 150
v=0
o=faxgw1 2890844527 2890844529 IN IP4 ift.here.com
s=Session SDP
c=IN IP4 iftmg.here.com
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F20 200 OK PROXY -> IFTGW UA
SIP/2.0 200 OK
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 150
v=0
o=faxgw1 2890844527 2890844529 IN IP4 ift.here.com
s=Session SDP
c=IN IP4 iftmg.here.com
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
Li/Mule 23
SIP T.38 Call Flows October 2001
F21 ACK IFTGW UA -> PROXY
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP iftgw.there.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 ACK
Content-Length: 0
F22 ACK PROXY -> IFT UA
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d009.1
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 ACK
Content-Length: 0
/* Voice streams re-established both ways */
F23 BYE IFT UA -> PROXY
BYE sip:+1-650-555-2222@obelix.wcom.com SIP/2.0
Via: SIP/2.0/UDP ift.here.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ift.here.com
CSeq: 20 BYE
Content-Length: 0
F24 BYE PROXY -> IFTGW UA
BYE sip:+1-650-555-2222@obelix.wcom.com SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d009.1
Via: SIP/2.0/UDP ift.here.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ift.here.com
Li/Mule 24
SIP T.38 Call Flows October 2001
CSeq: 20 BYE
Content-Length: 0
F25 200 OK IFTGW UA -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde24
Call-ID: 1717@ift.here.com
CSeq: 20 BYE
Content-Type: application/sdp
Content-Length: 0
F26 200 OK PROXY -> IFT UA
SIP/2.0 200 OK
Via: SIP/2.0/UDP ift.here.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde24
Call-ID: 1717@ift.here.com
CSeq: 20 BYE
Content-Type: application/sdp
Content-Length: 0
5.3 SIP T.38 fax call û fax stream added to voice stream
This section represents the SIP call flow for a T.38 fax session
between 2 Internet telephony gateways. It shows the detection of
the fax communication on terminating or receiving side.
A session starts with one audio stream in SDP, and, upon detection
of fax, an additional fax stream is added to the session while the
voice stream is muted. After the fax communication is terminated,
the fax stream is terminated and the voice stream is restored.
Scenario:
1. A SIP INVITE is sent to the called party requesting a voice
connection per RFC2543; our scenario involves 1 SIP proxy. An audio
connection is established,
2. Upon detection of Preamble by the terminating gateway, a SIP
re¡INVITE request is sent to the emitting gateway to modify the
parameters of the session to add a T.38 fax connection. The
connection data is specified per media description and not at the
session-level. This allows the audio stream and t.38 fax stream to
be controlled independently (i.e., placing the audio on hold while
the t.38 fax transmission is active). Also note that the t.38 fax
Li/Mule 25
SIP T.38 Call Flows October 2001
media stream has been added to the session. This re-INVITE request
contains SDP information detailing the additional T.38 capabilities,
3. It is suggested that the voice pipe be muted while the fax
session is in progress. Upon successful acknowledgments, T.38 IFP
fax packets are sent/received on different UDP ports than the one
used for audio RTP,
4. Once the fax transmission is terminated, audio capabilities are
ôrestoredö or the call is terminated.
5.3.1 Sequence Diagram
IFT UA Proxy IFTGW UA
| | |
| F1 INVITE | |
|------------------->| |
| | F2 INVITE |
| |------------------->|
| F3 100 Trying | |
|<-------------------| F4 100 Trying |
| |<-------------------|
| | |
| | F5 180 Ringing |
| F6 180 Ringing |<-------------------|
|<-------------------| |
| | F7 200 OK |
| F8 200 OK |<-------------------|
|<-------------------| |
| | |
| F9 ACK | |
|------------------->| F10 ACK |
| ------------------->|
| |
| Both Way RTP Media Established |
Fax |<=======================================>|
------->| |
emitted | | |
| | | Preamble
| | |<---------
| | F11 INVITE | detected
| |<-------------------|
| F12 INVITE | |
|<-------------------| |
| | |
| F13 200 OK | |
|------------------->| |
| | F14 200 OK |
| |------------------->|
| | F15 ACK |
| |<-------------------|
| F16 ACK | |
|<-------------------| |
Li/Mule 26
SIP T.38 Call Flows October 2001
| |
| T.38/UDPT Fax Flow Established |
| Voice connection muted |
|<=======================================>|
End of | |
fax | | |
detected| | |
------->| | |
| |
| | | End of fax
| | |<----------
| | | detected
| | F17 INVITE |
| |<-------------------|
| F18 INVITE | |
|<-------------------| |
| | |
| F19 200 OK | |
|------------------->| |
| | F20 200 OK |
| |------------------->|
| | |
| | F21 ACK |
| |<-------------------|
| F22 ACK | |
|<-------------------| |
| |
| Both way RTP Flow restored |
|<=======================================>|
| |
| F23 BYE | |
|------------------->| |
| | F24 BYE |
| |------------------->|
| | |
| | F25 200 OK |
| |<-------------------|
| F26 200 OK | |
|<-------------------| |
5.3.2 Message Details
F1 INVITE IFT UA -> PROXY
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Li/Mule 27
SIP T.38 Call Flows October 2001
Content-Type: application/sdp
Content-Length: 146
v=0
o=IFAXTERMINAL01 2890844527 2890844527 IN IP4 ift.here.com
s=Session SDP
c=IN IP4 iftmg.here.com
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F2 INVITE PROXY -> IFTGW UA
INVITE sip:+1-650-555-2222@iftgw.there.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ift.here.com:5060
Record-Route: <sip:+1-650-555-2222@iftgw.there.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 146
v=0
o=IFAXTERMINAL01 2890844527 2890844527 IN IP4 ift.here.com
s=Session SDP
c=IN IP4 iftmg.here.com
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F3 (100 Trying) PROXY -> IFT UA
SIP/2.0 100 Trying
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Content-Length: 0
F4 100 Trying IFTGW UA -> PROXY
SIP/2.0 100 Trying
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ift.here.com:5060
Li/Mule 28
SIP T.38 Call Flows October 2001
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Length: 0
F5 180 Ringing IFTGW UA -> PROXY
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Length: 0
F6 180 Ringing PROXY -> IFT UA
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Length: 0
F7 200 OK IFTGW UA -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 150
v=0
o=IFAXTERMINAL01 2890844527 171089 IN IP4 iftgw.there.com
s=Session SDP
c=IN IP4 iftmg.there.com
t=0 0
m=audio 12322 RTP/AVP 0
a=rtpmap:0 PCMU/8000
Li/Mule 29
SIP T.38 Call Flows October 2001
F8 200 OK PROXY -> IFT UA
SIP/2.0 200 OK
Via: SIP/2.0/UDP ift.here.com:5060
Record-Route: <sip:+1-650-555-2222@iftgw.there.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 17 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 150
v=0
o=IFAXTERMINAL01 2890844527 171089 IN IP4 iftgw.there.com
s=Session SDP
c=IN IP4 iftmg.there.com
t=0 0
m=audio 12322 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F9 ACK IFT UA -> PROXY
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP ift.here.com:5060
Route: <sip:+1-650-555-2222@iftgw.there.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 17 ACK
Content-Length: 0
F10 ACK PROXY -> IFTGW UA
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ift.here.com:5060
Record-Route: <sip:+1-650-555-2222@iftgw.there.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 17 ACK
Content-Length: 0
Li/Mule 30
SIP T.38 Call Flows October 2001
/* RTP streams are established. The CNG fax tone is sent in-band if
it is present. The receiving side IFT gateway DSP detects the
Preamble. A new UDP port is open on IFTGW for T.38 IFP packets and
the IFTGW signals the switch over to fax mode by send a re-INVITE
with the new UDP port in the SDP */
F11 INVITE IFTGW UA -> PROXY
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP iftgw.there.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 320
v=0
o=faxgw1 2890844527 171090 IN IP4 iftgw.there.com
s=Session SDP
t=0 0
m=audio 12322 RTP/AVP 0
c=IN IP4 0.0.0.0
m=image 49172 udptl t38
c=IN IP4 iftmg.there.com
a=T38FaxVersion:0
a=T38MaxBitRate:14400
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPFEC
a=T38FaxUdpEC:t38UDPRedundancy
F12 INVITE PROXY -> IFT UA
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d008.1
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Li/Mule 31
SIP T.38 Call Flows October 2001
Content-Type: application/sdp
Content-Length: 320
v=0
o=faxgw1 2890844527 171090 IN IP4 iftgw.there.com
s=Session SDP
t=0 0
m=audio 12322 RTP/AVP 0
c=IN IP4 0.0.0.0
m=image 49172 udptl t38
c=IN IP4 iftmg.there.com
a=T38FaxVersion:0
a=T38MaxBitRate:14400
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPFEC
a=T38FaxUdpEC:t38UDPRedundancy
F13 200 OK IFT UA -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d008.1
Via: SIP/2.0/UDP iftgw.there.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 320
v=0
o=faxgw1 2890846527 2890846528 IN IP4 ift.here.com
s=Session SDP
t=0 0
m=audio 3456 RTP/AVP 0
c=IN IP4 0.0.0.0
m=image 15002 udptl t38
c=IN IP4 iftmg.here.com
a=T38FaxVersion:0
a=T38MaxBitRate:14400
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPRedundancy
Li/Mule 32
SIP T.38 Call Flows October 2001
F14 200 OK PROXY -> IFT UA
SIP/2.0 200 OK
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 320
v=0
o=faxgw1 2890846527 2890846528 IN IP4 ift.here.com
s=Session SDP
t=0 0
m=audio 3456 RTP/AVP 0
c=IN IP4 0.0.0.0
m=image 15002 udptl t38
c=IN IP4 iftmg.here.com
a=T38FaxVersion:0
a=T38MaxBitRate:14400
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPRedundancy
F15 ACK IFTGW UA -> PROXY
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP iftgw.there.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 ACK
Content-Length: 0
F16 ACK PROXY -> IFT UA
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d008.1
Via: SIP/2.0/UDP iftgw.there.com:5060
Li/Mule 33
SIP T.38 Call Flows October 2001
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 ACK
Content-Length: 0
/* T.38 fax transmission established both ways and the voice stream
is muted */
/* Then, the end of the fax transmission is detected on ingress side
and sent to the egress side (IFTGW). IFTGW initiates the switch back
to voice communication */
F17 INVITE IFTGW UA -> PROXY
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP iftgw.there.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 181
v=0
o=faxgw1 2890844527 171091 IN IP4 iftgw.there.com
s=Session SDP
c=IN IP4 iftmg.there.com
t=0 0
m=audio 12322 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F18 INVITE PROXY -> IFT UA
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d009.1
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 181
Li/Mule 34
SIP T.38 Call Flows October 2001
v=0
o=faxgw1 2890844527 171091 IN IP4 iftgw.there.com
s=Session SDP
c=IN IP4 iftmg.there.com
t=0 0
m=audio 12322 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F19 200 OK IFT UA -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d008.1
Via: SIP/2.0/UDP iftgw.there.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 150
v=0
o=faxgw1 2890844527 2890844529 IN IP4 ift.here.com
s=Session SDP
c=IN IP4 iftmg.here.com
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F20 200 OK PROXY -> IFTGW UA
SIP/2.0 200 OK
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 150
v=0
o=faxgw1 2890844527 2890844529 IN IP4 ift.here.com
s=Session SDP
c=IN IP4 iftmg.here.com
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
Li/Mule 35
SIP T.38 Call Flows October 2001
F21 ACK IFTGW UA -> PROXY
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP iftgw.there.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 ACK
Content-Length: 0
F22 ACK PROXY -> IFT UA
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d009.1
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 ACK
Content-Length: 0
F23 BYE IFT UA -> PROXY
BYE sip:+1-650-555-2222@obelix.wcom.com SIP/2.0
Via: SIP/2.0/UDP ift.here.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ift.here.com
CSeq: 20 BYE
Content-Length: 0
F24 BYE PROXY -> IFTGW UA
BYE sip:+1-650-555-2222@obelix.wcom.com SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d009.1
Via: SIP/2.0/UDP ift.here.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone
Call-ID: 1717@ift.here.com
CSeq: 20 BYE
Content-Length: 0
Li/Mule 36
SIP T.38 Call Flows October 2001
F25 200 OK IFTGW UA -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde24
Call-ID: 1717@ift.here.com
CSeq: 20 BYE
Content-Type: application/sdp
Content-Length: 0
F26 200 OK PROXY -> IFT UA
SIP/2.0 200 OK
Via: SIP/2.0/UDP ift.here.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde24
Call-ID: 1717@ift.here.com
CSeq: 20 BYE
Content-Type: application/sdp
Content-Length: 0
5.4 SIP T.38 fax call û fax stream forced to replace voice stream
This section illustrates the following scenario:
1. A SIP INVITE is sent to the called party requesting a voice
connection per RFC2543; our scenario involves 1 SIP proxy. An audio
connection is established,
2. Upon detection of Preamble by the terminating gateway, a SIP
re¡INVITE request is sent to the emitting gateway to modify the
parameters of the session to add a T.38 fax connection. This re-
INVITE request contains SDP information detailing the additional
T.38 capabilities. It is also suggested that the voice pipe be
muted while the fax session is in progress.
3. The emitting gateway does not have the resources to maintain both
the audio & fax streams and replies by allowing the fax media
stream. Upon successful acknowledgments, the audio stream is
terminated and T.38 IFP fax packets are exchanged,
4. Once the fax transmission is terminated, audio capabilities are
ôre-negotiatedö and the call is terminated.
5.4.1 Sequence Diagram
Refer to section 5.3.1; sequence diagram is unchanged.
Li/Mule 37
SIP T.38 Call Flows October 2001
5.4.2 Message Details
See section 5.3.2 for the complete set of messages. We only
indicate the changed messages based on the emitting gatewayÆs
decline to support both voice and fax streams.
...
F11 INVITE IFTGW UA -> PROXY
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP iftgw.there.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 320
v=0
o=faxgw1 2890844527 171090 IN IP4 iftgw.there.com
s=Session SDP
t=0 0
m=audio 12322 RTP/AVP 0
c=IN IP4 0.0.0.0
m=image 49172 udptl t38
c=IN IP4 iftmg.there.com
a=T38FaxVersion:0
a=T38MaxBitRate:14400
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPFEC
a=T38FaxUdpEC:t38UDPRedundancy
F12 INVITE PROXY -> IFT UA
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d008.1
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Li/Mule 38
SIP T.38 Call Flows October 2001
Content-Length: 320
v=0
o=faxgw1 2890844527 171090 IN IP4 iftgw.there.com
s=Session SDP
t=0 0
m=audio 12322 RTP/AVP 0
c=IN IP4 0.0.0.0
m=image 49172 udptl t38
c=IN IP4 iftmg.there.com
a=T38FaxVersion:0
a=T38MaxBitRate:14400
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPFEC
a=T38FaxUdpEC:t38UDPRedundancy
F13 200 OK IFT UA -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d008.1
Via: SIP/2.0/UDP iftgw.there.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 320
v=0
o=faxgw1 2890846527 2890846528 IN IP4 ift.here.com
s=Session SDP
t=0 0
m=audio 0 RTP/AVP 0
c=IN IP4 0.0.0.0
m=image 15002 udptl t38
c=IN IP4 iftmg.here.com
a=T38FaxVersion:0
a=T38MaxBitRate:14400
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPRedundancy
Li/Mule 39
SIP T.38 Call Flows October 2001
F14 200 OK PROXY -> IFT UA
SIP/2.0 200 OK
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 320
v=0
o=faxgw1 2890846527 2890846528 IN IP4 ift.here.com
s=Session SDP
t=0 0
m=audio 0 RTP/AVP 0
c=IN IP4 0.0.0.0
m=image 15002 udptl t38
c=IN IP4 iftmg.here.com
a=T38FaxVersion:0
a=T38MaxBitRate:14400
a=T38FaxFillBitRemoval:0
a=T38FaxTranscodingMMR:0
a=T38FaxTranscodingJBIG:0
a=T38FaxRateManagement:transferredTCF
a=T38FaxMaxBuffer:72
a=T38FaxMaxDatagram:316
a=T38FaxUdpEC:t38UDPRedundancy
F15 ACK IFTGW UA -> PROXY
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP iftgw.there.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 ACK
Content-Length: 0
F16 ACK PROXY -> IFT UA
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d008.1
Via: SIP/2.0/UDP iftgw.there.com:5060
Li/Mule 40
SIP T.38 Call Flows October 2001
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 ACK
Content-Length: 0
6. Unsuccessful Scenarios
6.1. Unsuccessful T.38 fax scenario û no support for T.38
This section represents a SIP T.38 call failure because the emitting
gateway does not support T.38.
Scenario:
1. A SIP INVITE is sent to the called party requesting a voice
connection per RFC2543; our scenario involves 1 SIP proxy. An audio
connection is established,
2. Upon detection of Preamble by the terminating gateway, a SIP
re¡INVITE request is sent to the emitting gateway to modify the
parameters of the session to allow a T.38 fax connection. This re-
INVITE request contains SDP information detailing T.38 capabilities,
3. The emitting gateway rejects the re¡INVITE with a 415 Unsupported
Media Type response.
4. Upon reception of the reject, the call is terminated.
6.1.1. Sequence Diagram
The messages F1 to F12 are identical to section 5.2.
IFT UA Proxy IFTGW UA
| | |
| F1 INVITE | |
|------------------->| |
| | F2 INVITE |
| |------------------->|
| F3 100 Trying | |
|<-------------------| F4 100 Trying |
| |<-------------------|
| | |
| | F5 180 Ringing |
| F6 180 Ringing |<-------------------|
|<-------------------| |
| | F7 200 OK |
| F8 200 OK |<-------------------|
|<-------------------| |
| F9 ACK | |
|------------------->| F10 ACK |
| ------------------->|
| |
Li/Mule 41
SIP T.38 Call Flows October 2001
| Both Way RTP Media Established |
Fax |<=======================================>|
------->| |
emitted | | |
| | | Preamble
| | |<---------
| | F11 INVITE | detected
| |<-------------------|
| F12 INVITE | |
|<-------------------| |
| | |
| F13 415 | |
|------------------->| |
| | F14 415 |
| |------------------->|
| | |
| | F15 BYE |
| |<-------------------|
| F16 BYE | |
|<-------------------| |
| | |
| F17 200 OK | |
|------------------->| |
| | F18 200 OK |
| |------------------->|
6.1.2. Message Details
Messages F1 to F12 are identical to section 5.2; refer to 5.2.
F13 415 Unsupported Media Type IFT UA -> PROXY
SIP/2.0 415 Unsupported Media Type
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Length: 0
F14 415 Unsupported Media Type PROXY -> IFTGW UA
SIP/2.0 415 Unsupported Media Type
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d010.1
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Li/Mule 42
SIP T.38 Call Flows October 2001
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Length: 0
F15 BYE IFTGW UA -> PROXY
BYE sip:+1-650-555-2222@obelix.wcom.com SIP/2.0
Via: SIP/2.0/UDP iftgw.there.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde34
Call-ID: 1717@ift.here.com
CSeq: 19 BYE
Content-Length: 0
F16 BYE PROXY -> IFT UA
BYE sip:+1-650-555-2222@obelix.wcom.com SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d010.1
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde34
Call-ID: 1717@ift.here.com
CSeq: 19 BYE
Content-Length: 0
F17 200 OK IFT UA -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde34
Call-ID: 1717@ift.here.com
CSeq: 19 BYE
Content-Type: application/sdp
Content-Length: 0
F18 200 OK PROXY -> IFTGW UA
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.wcom.com:5060
Via: SIP/2.0/UDP ift.here.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde34
Call-ID: 1717@ift.here.com
Li/Mule 43
SIP T.38 Call Flows October 2001
CSeq: 19 BYE
Content-Type: application/sdp
Content-Length: 0
6.2. Unsuccessful T.38 fax scenario û 606 response and G.711 fallback
This section represents am unsuccessful SIP T.38 fax call: the
emitting gateway does not support T.38 and responds with a 606 Not
Acceptable to indicate that some aspects of the session description
are not acceptable. The terminating gateway may react by proposing
a fallback to G.711 fax pass-through with special codec
characteristics û echo cancellation ON, silence suppression OFF.
The message details in this section make use of the generic SDP
attributes ecan and silenceSupp, defined in RFC3108 [10].
Scenario:
1. A SIP INVITE is sent to the called party requesting a voice
connection per RFC2543; our scenario involves 1 SIP proxy. An audio
connection is established,
2. Upon detection of Preamble by the terminating gateway, a SIP
re¡INVITE request is sent to the emitting gateway to modify the
parameters of the session to allow a T.38 fax connection. This re-
INVITE request contains SDP information detailing T.38 capabilities,
3. The emitting gateway rejects the re-INVITE with a 606 Not
Acceptable response. Upon reception of the reject, the terminating
gateway proposes to use plain G.711 to carry the fax stream.
4. The call is terminated.
6.2.1. Sequence Diagram
The messages F1 to F12 are identical to section 5.2.
IFT UA Proxy IFTGW UA
| | |
| F1 INVITE | |
|------------------->| |
| | F2 INVITE |
| |------------------->|
| F3 100 Trying | |
|<-------------------| F4 100 Trying |
| |<-------------------|
| | |
| | F5 180 Ringing |
| F6 180 Ringing |<-------------------|
|<-------------------| |
| | F7 200 OK |
| F8 200 OK |<-------------------|
|<-------------------| |
| F9 ACK | |
|------------------->| F10 ACK |
| ------------------->|
| |
Li/Mule 44
SIP T.38 Call Flows October 2001
| Both Way RTP Media Established |
Fax |<=======================================>|
------->| |
emitted | | |
| | | Preamble
| | |<---------
| | F11 INVITE | detected
| |<-------------------|
| F12 INVITE | |
|<-------------------| |
| | |
| F13 606 | |
|------------------->| |
| | F14 606 |
| |------------------->|
| | F15 INVITE |
| |<-------------------|
| F16 INVITE | |
|<-------------------| |
| F17 200 OK | |
|------------------->| |
| | F18 200 OK |
| |------------------->|
| | |
| | F19 ACK |
| |<-------------------|
| F20 ACK | |
|<-------------------| |
| |
| G.711 RTP Flow Established |
|<=======================================>|
| for fax pass-through |
| |
| F21 BYE | |
|------------------->| |
| | F22 BYE |
| |------------------->|
| | |
| | F23 200 OK |
| |<-------------------|
| F24 200 OK | |
|<-------------------| |
6.2.2. Message Details
Messages F1 to F12 are identical to section 5.2; refer to 5.2.
F13 606 Not Acceptable IFT UA -> PROXY
SIP/2.0 606 Not Acceptable
Via: SIP/2.0/UDP ift.here.com:5060
Li/Mule 45
SIP T.38 Call Flows October 2001
Warning: 304 Media type not available: One or more media types
contained in the session description are not available.
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Content-Length: 0
F14 606 Not Acceptable PROXY -> IFTGW UA
SIP/2.0 606 Not Acceptable
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d010.1
Via: SIP/2.0/UDP ift.here.com:5060
Warning: 304 Media type not available: One or more media types
contained in the session description are not available.
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 18 INVITE
Content-Length: 0
F15 INVITE IFTGW UA -> PROXY
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP iftgw.there.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 181
v=0
o=faxgw1 2890844527 171091 IN IP4 iftgw.there.com
s=Session SDP
c=IN IP4 iftmg.there.com
t=0 0
m=audio 12322 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=ecan:fb on -
a=silenceSupp:off - - -
F16 INVITE PROXY -> IFT UA
INVITE sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d009.1
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
Li/Mule 46
SIP T.38 Call Flows October 2001
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 INVITE
Contact: <sip:+1-650-555-2222@iftgw.there.com>
Content-Type: application/sdp
Content-Length: 181
v=0
o=faxgw1 2890844527 171091 IN IP4 iftgw.there.com
s=Session SDP
c=IN IP4 iftmg.there.com
t=0 0
m=audio 12322 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=ecan:fb on -
a=silenceSupp:off - - -
F17 200 OK IFT UA -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d008.1
Via: SIP/2.0/UDP iftgw.there.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 150
v=0
o=faxgw1 2890844527 2890844528 IN IP4 ift.here.com
s=Session SDP
c=IN IP4 iftmg.here.com
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=ecan:fb on -
a=silenceSupp:off - - -
F18 200 OK PROXY -> IFTGW UA
SIP/2.0 200 OK
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
Li/Mule 47
SIP T.38 Call Flows October 2001
CSeq: 19 INVITE
Contact: <sip:+1-303-555-1111@ift.here.com>
Content-Type: application/sdp
Content-Length: 150
v=0
o=faxgw1 2890844527 2890844528 IN IP4 ift.here.com
s=Session SDP
c=IN IP4 iftmg.here.com
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=ecan:fb on -
a=silenceSupp:off - - -
F19 ACK IFTGW UA -> PROXY
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP iftgw.there.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 ACK
Content-Length: 0
F20 ACK PROXY -> IFT UA
ACK sip:+1-650-555-2222@obelix.wcom.com;user=phone SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d009.1
Via: SIP/2.0/UDP iftgw.there.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab11
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde17
Call-ID: 1717@ift.here.com
CSeq: 19 ACK
Content-Length: 0
/* G.711 audio streams established both ways for fax pass through */
F21 BYE IFT UA -> PROXY
BYE sip:+1-650-555-2222@obelix.wcom.com SIP/2.0
Via: SIP/2.0/UDP ift.here.com:5060
Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
Li/Mule 48
SIP T.38 Call Flows October 2001
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde34
Call-ID: 1717@ift.here.com
CSeq: 20 BYE
Content-Length: 0
F22 BYE PROXY -> IFTGW UA
BYE sip:+1-650-555-2222@obelix.wcom.com SIP/2.0
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d009.1
Via: SIP/2.0/UDP ift.here.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde34
Call-ID: 1717@ift.here.com
CSeq: 20 BYE
Content-Length: 0
F23 200 OK IFTGW UA -> PROXY
SIP/2.0 200 OK
Via: SIP/2.0/UDP obelix.wcom.com:5060; branch=2d007.1
Via: SIP/2.0/UDP ift.here.com:5060
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde34
Call-ID: 1717@ift.here.com
CSeq: 20 BYE
Content-Type: application/sdp
Content-Length: 0
F24 200 OK PROXY -> IFT UA
SIP/2.0 200 OK
Via: SIP/2.0/UDP ift.here.com:5060
Record-Route: <sip:+1-650-555-2222@obelix.wcom.com;
maddr=obelix.wcom.com>
From: sip:+1-303-555-1111@ift.here.com;user=phone;tag=ab22
To: sip:+1-650-555-2222@obelix.wcom.com;user=phone;tag=cde34
Call-ID: 1717@ift.here.com
CSeq: 20 BYE
Content-Type: application/sdp
Content-Length: 0
Li/Mule 49
SIP T.38 Call Flows October 2001
7. SDP Attribute Table for T.38 sessions
For a detailed description of these attributes, refer to IANA. The
tables below are replicated here for reference only.
+-----------------------+---------------------+--------------+
| SDP Attribute Name | Appropriate values | Example |
| ("att-field") | | |
+-----------------------+---------------------+--------------+
| | | |
| T38FaxVersion | 1*DIGIT | 0 |
| | | |
| T38MaxBitRate | 1*(DIGIT) | 14400 |
| | | |
| T38FaxFillBitRemoval | boolean | 0 |
| | | |
| T38FaxTranscodingMMR | boolean | 0 |
| | | |
| T38FaxTranscodingJBIG | boolean | 0 |
| | | |
| T38FaxRateManagement | localTCF | |transferredTCF|
| | transferredTCF | |
| | | |
| T38FaxMaxBuffer | 1*(DIGIT); optional | 72 |
| | | (bytes) |
| | | |
| T38FaxMaxDatagram | 1*(DIGIT); | Depends on |
| | optional | redundancy; |
| | | 316 (bytes)|
| | | |
| T38FaxUdpEC | t38UDPFEC | | T38UDPRedund |
| | t38UDPRedundancy | ancy |
+-----------------------+---------------------+--------------+
Registered SDP Protocol ôprotoö for T.38:
+-----------------+
| Name |
| |
+-----------------+
| |
| UDPTL |
| |
| TCP |
| |
+-----------------+
Registered SDP Protocol ôfmtö, MIME media type image/t38:
MIME media type name: image
MIME subtype name: t38
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SIP T.38 Call Flows October 2001
8. Considerations
In this temporary section, we log the open items for discussion as
well as issues that have been solved on the mailing list:
8.1. Level of requirements for Internet gateways
--- Status: closed, solved by draft01 text in section 4.2
Based on the comments received on draft-00, the authors recommend
that: T.38 Internet fax gateways MUST trigger a SIP re-INVITE upon
detection of the V.21 Preamble flag sequence on the receiving side.
Emitting gateways MAY trigger session modification upon CNG.
8.2. Negotiation of UDP ports for T.38 transmission
--- Status: not an issue, just logged here
When switching to T.38 mode, Internet fax gateways specify a new set
of parameters for the media connection in SDP. Some gateways may
choose the UDP/RTP port used in audio mode for UDP/UDPTL/T.38
traffic. Some gateways may choose to open a new UDP port for
UDPTL/T.38 fax (and keep the previous UDP/RTP port open for the
switch back to audio).
SDP provides the mechanism to precisely define in SIP all cases:
- if an Internet Fax gateway desires to re-use the UDP/RTP port for
UDP/UDPTL/T.38, put 1 ôm=ö line in the re-INVITE.
m=image <original_RTP_port> udptl t38
- if an Internet Fax gateway desires to release the RTP port and
open a new UDP port for T.38:
m=image <new_UDP_port> udptl t38
- if an Internet Fax gateway desires to keep the RTP port for future
use in the session and open a new UDP port for T.38, we need to
repeat the RTP/AVP port:
m=audio <original_RTP_port> RTP/AVP 0
m=image <new_UDP_port> udptl t38
8.3. Adding a fax stream to SIP session vs. Replacing audio stream
--- Status: closed,see flows 5.2 and 5.3
Based on the mailing list discussion, it is recommended in section
5.2 to replace an existing audio media connection with a fax one.
We however kept section 5.3 to illustrate what adding a fax stream
would entail.
8.4. Adding text to make use of SIP OPTIONS: in progress in draft-02
--- Status: Open for discussion
As discussed with Bill Sulzen, the SIP OPTIONS response could be
used to assert that real-time fax communications using either T.38
or fax-passthrough are supported. This could certainly improve the
negotiation phases and reduce the number of SIP messages. It is
also key to get the capabilities of the Internet-aware fax device or
gateway other than on a per call basis.
What needs to be added:
- add text in sub-section in section 4.5
- specify how the Accept header line should be populated (do we
want to specify t.38 and/or fax pass-through (meaning ability to
Li/Mule 51
SIP T.38 Call Flows October 2001
support ecan and silencesuppression), and within t.38, do we want to
specify whether it is t.38 over tcp, udptl, both, etc.
8.5. Use of IP addresses in SDP connection information
--- Status: Open for discussion
Comment from Christer Holmberg: SDP connection information should
use IP addresses instead of domain names (as per draft-ietf-mmusic-
sdp-new-03)
8.6. Unsuccessful scenario: 606 or 488
--- Status: Open for discussion
Comment from Bill Sulzen: it is acceptable or even preferable to
reject with a 488 Not Acceptable Media?
9. Security Considerations
The security mechanisms provided in RFC2543 apply: message
authentication can be performed on SIP INVITEs and also BYE.
10. References
1 S. Bradner, "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2 S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
3 M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
Session Initiation Protocol", RFC 2543, March 1999.
4 ITU-T Recommendation T.38, ôProcedures for real-time Group 3
facsimile communication over IP networksö, June 1998.
5 Johnston, et al, ôSIP Telephony Call Flow Examplesö, Internet-
Draft draft-ietf-sip-call-flows-05.txt, June 2001.
6 ITU-T Recommendation T.38 Amendment 2, ôProcedures for real-time
Group 3 facsimile communication over IP networksö, February 2000.
7 ITU-T Recommendation T.30, ôProcedures for document facsimile
transmission in the general switched telephone networkö, March
1993.
8 M. Handley, V. Jacobson, "SDP: session description protocol",
RFC 2327, April 1998.
9 ITU-T Recommendation T.38 Amendment 2 Annex D, ôSIP/SDP Call
Establishment Proceduresö, February 2000.
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SIP T.38 Call Flows October 2001
10 R. Kumar, M. Mostafa, ôConventions for the use of the Session
Description Protocol (SDP) for ATM Bearer Connectionsö, RFC3108,
May 2001.
11. Acknowledgments
This document would not have been possible without the help of the
following development engineers and fax experts, in particular
Stanley Khouw, Bill Michalek, and George Maphis of Clarent
Corporation.
The authors would like to thank Bill Sulzen of Cisco Systems for his
continued support and critical comments. We also acknowledge the
valuable contributions from Flemming Andreasen of Cisco Systems,
Christer Holmberg of Ericsson, and Bert Culpepper of InterVoice-
Brite.
Many thanks to the reviewers of the previous document for their
comments: Jim Renkel of 3COM, Sudipto Mukherjee of Cisco Systems,
Arjun Roychowdhury of Hughes Software Systems.
12. Author's Addresses
Jieying Li
Clarent Corporation
700 Chesapeake Drive MS 101
Redwood City, CA 94063
email: jieying.li@clarent.com
Jean-Francois MuleÆ
email: jfmule@packetizer.com
Full Copyright Statement
"Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into
Li/Mule 53