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Versions: 00 01 02

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

Li, Mule                        SIP WG                               1

                         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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                         SIP T.38 Call Flows             October 2001


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

Li/Mule                                                              3

                         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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                         SIP T.38 Call Flows             October 2001


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).

Li/Mule                                                              5

                         SIP T.38 Call Flows             October 2001



   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

Li/Mule                                                              6

                         SIP T.38 Call Flows             October 2001


   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   |
           |                    |<-------------------|
           |                    |                    |


Li/Mule                                                              7

                         SIP T.38 Call Flows             October 2001


           |                    |   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

Li/Mule                                                              8

                         SIP T.38 Call Flows             October 2001


      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






Li/Mule                                                              9

                         SIP T.38 Call Flows             October 2001


   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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                         SIP T.38 Call Flows             October 2001


      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

Li/Mule                                                             11

                         SIP T.38 Call Flows             October 2001


      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






Li/Mule                                                             12

                         SIP T.38 Call Flows             October 2001


   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

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                         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          |
           |                     ------------------->|
           |                                         |


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                         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

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                         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;

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                         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

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                         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

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                         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









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                         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

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                         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


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