path: root/doc/rfc/rfc7088.txt

Internet Engineering Task Force (IETF)                         D. Worley
Request for Comments: 7088                                       Ariadne
Category: Informational                                    February 2014
ISSN: 2070-1721


      Session Initiation Protocol Service Example -- Music on Hold

Abstract

   "Music on hold" is one of the features of telephone systems that is
   most desired by buyers of business telephone systems.  Music on hold
   means that when one party to a call has the call "on hold", that
   party's telephone provides an audio stream (often music) to be heard
   by the other party.  Architectural features of SIP make it difficult
   to implement music on hold in a way that is fully standards-
   compliant.  The implementation of music on hold described in this
   document is fully effective, is standards-compliant, and has a number
   of advantages over the methods previously documented.  In particular,
   it is less likely to produce peculiar user interface effects and more
   likely to work in systems that perform authentication than the music-
   on-hold method described in Section 2.3 of RFC 5359.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7088.













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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





































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Table of Contents

   1. Introduction ....................................................4
      1.1. Requirements Language ......................................4
   2. Technique .......................................................4
      2.1. Placing a Call on Hold and Establishing an External
           Media Stream ...............................................5
      2.2. Taking a Call off Hold and Terminating the External
           Media Stream ...............................................6
      2.3. Example Message Flow .......................................6
      2.4. Receiving Re-INVITE and UPDATE from the Remote UA .........17
      2.5. Receiving INVITE with Replaces ............................17
      2.6. Receiving REFER from the Remote UA ........................19
      2.7. Receiving Re-INVITE and UPDATE from the
           Music-on-Hold Source ......................................21
      2.8. Handling Payload Type Numbers .............................22
           2.8.1. Analysis ...........................................22
           2.8.2. Solution to the Problem ............................23
           2.8.3. Example of the Solution ............................24
      2.9. Dialog/Session Timers .....................................28
      2.10. When the Media Stream Directionality is "inactive" .......28
      2.11. Multiple Media Streams ...................................28
   3. Advantages .....................................................29
   4. Caveats ........................................................30
      4.1. Offering All Available Media Formats ......................30
      4.2. Handling Re-INVITES in a B2BUA ............................31
   5. Security Considerations ........................................31
      5.1. Network Security ..........................................31
      5.2. SIP (Signaling) Security ..................................32
      5.3. RTP (Media) Security ......................................32
      5.4. Media Filtering ...........................................32
   6. Acknowledgments ................................................33
   7. References .....................................................34
      7.1. Normative References ......................................34
      7.2. Informative References ....................................34
















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1.  Introduction

   Within systems based on SIP [RFC3261], it is desirable to be able to
   provide features that are similar to those provided by traditional
   telephony systems.  A frequently requested feature is "music on
   hold": with this feature, when one party to a call has the call "on
   hold", that party's telephone provides an audio stream (often music)
   to be heard by the other party.

   Architectural features of SIP make it difficult to implement music on
   hold in a way that is fully standards-compliant.  The purpose of this
   document is to describe a method that is reasonably simple yet fully
   effective and standards-compliant.  This method has significant
   advantages over other methods now in use, as described in Section 3.

   All current methods of implementing music on hold interoperate with
   each other, in that the two user agents in a call can use different
   methods for implementing music on hold with the same functionality as
   if either of the methods was used by both user agents.  Thus, there
   is no loss of functionality if different music-on-hold methods are
   used by different user agents within a telephone system or if a
   single user agent uses different methods within different calls or at
   different times within one call.

1.1.  Requirements Language

   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 [RFC2119].

2.  Technique

   The essence of the technique is that when the executing user agent
   (UA) (the user's UA) performs a re-INVITE of the remote UA (the other
   user's UA) to establish the hold state, it provides no Session
   Description Protocol (SDP) [RFC4566] offer [RFC3264] [RFC6337], thus
   compelling the remote UA to provide an SDP offer.  The executing UA
   then extracts the offer SDP from the remote UA's 2xx response and
   uses that as the offer SDP in a new INVITE to the external media
   source.  The external media source is thus directed to provide media
   directly to the remote UA.  The media source's answer SDP is returned
   to the remote UA in the ACK to the re-INVITE.









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2.1.  Placing a Call on Hold and Establishing an External Media Stream

   1.  The executing user instructs the executing UA to put the dialog
       on hold.

   2.  The executing UA sends a re-INVITE without SDP to the remote UA,
       which forces the remote UA to provide an SDP offer in its 2xx
       response.  The Contact header of the re-INVITE includes the
       '+sip.rendering="no"' field parameter to indicate that it is
       putting the call on hold ([RFC4235], Section 5.2).

   3.  The remote UA sends a 2xx to the re-INVITE and includes an SDP
       offer giving its own listening address/port.  If the remote UA
       understands the sip.rendering feature parameter, the offer may
       indicate that it will not send media by specifying the media
       directionalities as "recvonly" (the reverse of "on hold") or
       "inactive".  But the remote UA may offer to send media.

   4.  The executing UA uses this offer to derive the offer SDP of an
       initial INVITE that it sends to the configured music-on-hold
       (MOH) source.  The SDP in this request is largely copied from the
       SDP returned by the remote UA in the previous step, particularly
       regarding the provided listening address/port and payload type
       numbers.  But the media directionalities are restricted to
       "recvonly" or "inactive" as appropriate.  The executing UA may
       want or need to change the "o=" line.  In addition, some
       "a=rtpmap" lines may need to be added to control the assignment
       of RTP payload type numbers (Section 2.8).

   5.  The MOH source sends a 2xx response to the INVITE, which contains
       an SDP answer that should include its media source address as its
       listening address/port.  This SDP must necessarily specify
       "sendonly" or "inactive" as the directionality for all media
       streams [RFC3264].

       Although this address/port should receive no RTP, the specified
       port determines the port for receiving the RTP Control Protocol
       (RTCP) (and conventionally, for sending RTCP [RFC4961]).

       By convention, UAs use their declared RTP listening ports as
       their RTP source ports as well [RFC4961].  The answer SDP will
       reach the remote UA, thus informing it of the address/port from
       which the MOH media will come and presumably preventing the
       remote UA from ignoring the MOH media if the remote UA filters
       media packets based on the source address.  This functionality
       requires the SDP answer to contain the sending address in the
       "c=" line, even though the MOH source does not receive RTP.




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   6.  The executing UA sends this SDP answer as its SDP answer in the
       ACK for the re-INVITE to the remote UA.  The "o=" line in the
       answer must be modified to be within the sequence of "o=" lines
       previously generated by the executing UA in the dialog.  Any
       dynamic payload type number assignments that have been created in
       the answer must be recorded in the state of the original dialog.

   7.  Due to the sip.rendering feature parameter in the Contact header
       of the re-INVITE and the media directionality in the SDP answer
       contained in the ACK, the on-hold state of the dialog is
       established (at the executing end).

   8.  After this point, the MOH source generates RTP containing the
       music-on-hold media and sends it directly to the listening
       address/port of the remote UA.  The executing UA maintains two
       dialogs (one to the remote UA, one to the MOH source) but does
       not see or handle the MOH RTP.

2.2.  Taking a Call off Hold and Terminating the External Media Stream

   1.  The executing user instructs the executing UA to take the dialog
       off hold.

   2.  The executing UA sends a re-INVITE to the remote UA with SDP that
       requests to receive media.  The Contact header of the re-INVITE
       does not include the '+sip.rendering="no"' field parameter.  (It
       may contain a sip.rendering field parameter with value "yes" or
       "unknown", or it may omit the field parameter.)  Thus, this
       re-INVITE removes the on-hold state of the dialog (at the
       executing end).  (Note that the version in "o=" line of the
       offered SDP must account for the SDP versions that were passed
       through from the MOH source.  Also note that any payload type
       numbers that were assigned in SDP provided by the MOH source must
       be respected.)

   3.  When the remote UA sends a 2xx response to the re-INVITE, the
       executing UA sends a BYE request in the dialog to the MOH source.

   4.  After this point, the MOH source does not generate RTP and
       ordinary RTP flow is reestablished in the original dialog.

2.3.  Example Message Flow

   This section shows a message flow that is an example of this
   technique.  The scenario is as follows.  Alice establishes a call
   with Bob.  Bob then places the call on hold, with music on hold
   provided from an external source.  Bob then takes the call off hold.
   In this scenario, Bob's user agent is the executing UA, while Alice's



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   UA is the remote UA.  Note that this is just one possible message
   flow that illustrates this technique; numerous variations on these
   operations are allowed by the applicable standards.

   Alice             Bob       Music Source

   Alice establishes the call:

     |                |              |
     |    INVITE F1   |              |
     |--------------->|              |
     | 180 Ringing F2 |              |
     |<---------------|              |
     |    200 OK F3   |              |
     |<---------------|              |
     |     ACK F4     |              |
     |--------------->|              |
     |       RTP      |              |
     |<==============>|              |
     |                |              |

   Bob places Alice on hold, compelling Alice's UA to provide SDP:

     |                |              |
     |   INVITE F5    |              |
     |   (no SDP)     |              |
     |<---------------|              |
     |   200 OK F6    |              |
     |   (SDP offer)  |              |
     |--------------->|              |
     |                |              |

   Bob's UA initiates music on hold:

     |                |              |
     |                |  INVITE F7   |
     |                |  (SDP offer, |
     |                |   rev. hold) |
     |                |------------->|
     |                | 200 OK F8    |
     |                | (SDP answer, |
     |                |  hold)       |
     |                |<-------------|
     |                |    ACK F9    |
     |                |------------->|
     |                |              |





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   Bob's UA provides an SDP answer containing the address/port
   of Music Source:

     |                |              |
     | ACK F10        |              |
     | (SDP answer,   |              |
     |  hold)         |              |
     |<---------------|              |
     |    no RTP      |              |
     |<..............>|              |
     |     Music-on-hold RTP         |
     |<==============================|
     |                |              |

   The music on hold is active.

   Bob takes Alice off hold:

     |                |              |
     |  INVITE F11    |              |
     |  (SDP offer)   |              |
     |<---------------|              |
     |   200 OK F12   |              |
     |   (SDP answer) |              |
     |--------------->|              |
     |     ACK F13    |              |
     |<---------------|              |
     |                |    BYE F14   |
     |                |------------->|
     |                |    200 F15   |
     |                |<-------------|
     |       RTP      |              |
     |<==============>|              |
     |                |              |

   The normal media session between Alice and Bob is resumed.















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   /* Alice calls Bob. */

   F1 INVITE Alice -> Bob

   INVITE sips:bob@biloxi.example.com SIP/2.0
   Via: SIP/2.0/TLS atlanta.example.com:5061
    ;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: Alice <sips:alice@atlanta.example.com>;tag=1234567
   To: Bob <sips:bob@biloxi.example.com>
   Call-ID: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   Contact: <sips:a8342043f@atlanta.example.com;gr>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, gruu
   Content-Type: application/sdp
   Content-Length: [omitted]

   v=0
   o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
   s=
   c=IN IP4 atlanta.example.com
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   F2 180 Ringing Bob -> Alice

   SIP/2.0 180 Ringing
   Via: SIP/2.0/TLS atlanta.example.com:5061
    ;branch=z9hG4bK74bf9
    ;received=192.0.2.103
   From: Alice <sips:alice@atlanta.example.com>;tag=1234567
   To: Bob <sips:bob@biloxi.example.com>;tag=23431
   Call-ID: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   Contact: <sips:bob@biloxi.example.com>
   Content-Length: 0













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   F3 200 OK Bob -> Alice

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS atlanta.example.com:5061
    ;branch=z9hG4bK74bf9
    ;received=192.0.2.103
   From: Alice <sips:alice@atlanta.example.com>;tag=1234567
   To: Bob <sips:bob@biloxi.example.com>;tag=23431
   Call-ID: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   Contact: <sips:bob@biloxi.example.com>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: [omitted]

   v=0
   o=bob 2890844527 2890844527 IN IP4 biloxi.example.com
   s=
   c=IN IP4 biloxi.example.com
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F4 ACK Alice -> Bob

   ACK sips:bob@biloxi.example.com SIP/2.0
   Via: SIP/2.0/TLS atlanta.example.com:5061
    ;branch=z9hG4bK74bfd
   Max-Forwards: 70
   From: Alice <sips:alice@atlanta.example.com>;tag=1234567
   To: Bob <sips:bob@biloxi.example.com>;tag=23431
   Call-ID: 12345600@atlanta.example.com
   CSeq: 1 ACK
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Length: 0













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   /* Bob places Alice on hold. */

   /* The re-INVITE contains no SDP, thus compelling Alice's UA
      to provide an offer. */

   F5 INVITE Bob -> Alice

   INVITE sips:a8342043f@atlanta.example.com;gr SIP/2.0
   Via: SIP/2.0/TLS biloxi.example.com:5061
    ;branch=z9hG4bK874bk
   To: Alice <sips:alice@atlanta.example.com>;tag=1234567
   From: Bob <sips:bob@biloxi.example.com>;tag=23431
   Call-ID: 12345600@atlanta.example.com
   CSeq: 712 INVITE
   Contact: <sips:bob@biloxi.example.com>;+sip.rendering="no"
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Length: 0

   /* Alice's UA provides an SDP offer.
      Since it does not know that it is being put on hold,
      the offer is the same as the original offer and describes
      bidirectional media. */

   F6 200 OK Alice -> Bob

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS biloxi.example.com:5061
    ;branch=z9hG4bK874bk
    ;received=192.0.2.105
   To: Alice <sips:alice@atlanta.example.com>;tag=1234567
   From: Bob <sips:bob@biloxi.example.com>;tag=23431
   Call-ID: 12345600@atlanta.example.com
   CSeq: 712 INVITE
   Contact: <sips:a8342043f@atlanta.example.com;gr>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, gruu
   Content-Type: application/sdp
   Content-Length: [omitted]

   v=0
   o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
   s=
   c=IN IP4 atlanta.example.com
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=active



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   /* Bob's UA initiates music on hold. */

   /* This INVITE contains Alice's offer, but with the media
      direction set to "reverse hold", receive-only. */

   F7 INVITE Bob -> Music Source

   INVITE sips:music@source.example.com SIP/2.0
   Via: SIP/2.0/TLS biloxi.example.com:5061
    ;branch=z9hG4bKnashds9
   Max-Forwards: 70
   From: Bob <sips:bob@biloxi.example.com>;tag=02134
   To: Music Source <sips:music@source.example.com>
   Call-ID: 4802029847@biloxi.example.com
   CSeq: 1 INVITE
   Contact: <sips:bob@biloxi.example.com>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, gruu
   Content-Type: application/sdp
   Content-Length: [omitted]

   v=0
   o=bob 2890844534 2890844534 IN IP4 atlanta.example.com
   s=
   c=IN IP4 atlanta.example.com
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=recvonly






















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   F8 200 OK Music Source -> Bob

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS biloxi.example.com:5061
    ;branch=z9hG4bKnashds9
    ;received=192.0.2.105
   From: Bob <sips:bob@biloxi.example.com>;tag=02134
   To: Music Source <sips:music@source.example.com>;tag=56323
   Call-ID: 4802029847@biloxi.example.com
   Contact: <sips:music@source.example.com>;automaton
        ;+sip.byeless;+sip.rendering="no"
   CSeq: 1 INVITE
   Content-Length: [omitted]

   v=0
   o=MusicSource 2890844576 2890844576 IN IP4 source.example.com
   s=
   c=IN IP4 source.example.com
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly


   F9 ACK Bob -> Music Source

   ACK sips:music@source.example.com SIP/2.0
   Via: SIP/2.0/TLS source.example.com:5061
    ;branch=z9hG4bK74bT6
   From: Bob <sips:bob@biloxi.example.com>;tag=02134
   To: Music Source <sips:music@source.example.com>;tag=56323
   Max-Forwards: 70
   Call-ID: 4802029847@biloxi.example.com
   CSeq: 1 ACK
   Content-Length: 0


   /* Bob's UA now sends the ACK that completes the re-INVITE
      to Alice and completes the SDP offer/answer.
      The ACK contains the SDP received from Music Source and thus
      contains the address/port from which Music Source will send media,
      and implies the address/port that Music
      Source will use to send/receive RTCP. */








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   F10 ACK Bob -> Alice

   ACK sips:a8342043f@atlanta.example.com;gr SIP/2.0
   Via: SIP/2.0/TLS biloxi.example.com:5061
    ;branch=z9hG4bKq874b
   To: Alice <sips:alice@atlanta.example.com>;tag=1234567
   From: Bob <sips:bob@biloxi.example.com>;tag=23431
   Call-ID: 12345600@atlanta.example.com
   CSeq: 712 ACK
   Contact: <sips:bob@biloxi.example.com>;+sip.rendering="no"
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Length: [omitted]

   v=0
   o=bob 2890844527 2890844528 IN IP4 biloxi.example.com
   s=
   c=IN IP4 source.example.com
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly

   /* Bob picks up the call by sending a re-INVITE to Alice. */


   F11 INVITE Bob -> Alice

   INVITE sips:a8342043f@atlanta.example.com;gr SIP/2.0
   Via: SIP/2.0/TLS biloxi.example.com:5061
    ;branch=z9hG4bK874bk
   To: Alice <sips:alice@atlanta.example.com>;tag=1234567
   From: Bob <sips:bob@biloxi.example.com>;tag=23431
   Call-ID: 12345600@atlanta.example.com
   CSeq: 713 INVITE
   Contact: <sips:bob@biloxi.example.com>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: [omitted]

   v=0
   o=bob 2890844527 2890844529 IN IP4 biloxi.example.com
   s=
   c=IN IP4 biloxi.example.com
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000



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   F12 200 OK Alice -> Bob

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS biloxi.example.com:5061
    ;branch=z9hG4bK874bk
    ;received=192.0.2.105
   To: Alice <sips:alice@atlanta.example.com>;tag=1234567
   From: Bob <sips:bob@biloxi.example.com>;tag=23431
   Call-ID: 12345600@atlanta.example.com
   CSeq: 713 INVITE
   Contact: <sips:a8342043f@atlanta.example.com;gr>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, gruu
   Content-Type: application/sdp
   Content-Length: [omitted]

   v=0
   o=alice 2890844526 2890844527 IN IP4 atlanta.example.com
   s=
   c=IN IP4 atlanta.example.com
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F13 ACK Bob -> Alice

   ACK sips:a8342043f@atlanta.example.com;gr SIP/2.0
   Via: SIP/2.0/TLS biloxi.example.com:5061
    ;branch=z9hG4bKq874b
   To: Alice <sips:alice@atlanta.example.com>;tag=1234567
   From: Bob <sips:bob@biloxi.example.com>;tag=23431
   Call-ID: 12345600@atlanta.example.com
   CSeq: 713 ACK
   Contact: <sips:bob@biloxi.example.com>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Length: 0













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RFC 7088                      Music on Hold                February 2014


   F14 BYE Bob -> Music Source

   BYE sips:music@source.example.com SIP/2.0
   Via: SIP/2.0/TLS biloxi.example.com:5061
    ;branch=z9hG4bK74rf
   Max-Forwards: 70
   From: Bob <sips:bob@biloxi.example.com>;tag=02134
   To: Music Source <sips:music@source.example.com>;tag=56323
   Call-ID: 4802029847@biloxi.example.com
   CSeq: 2 BYE
   Contact: <sips:bob@biloxi.example.com>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, gruu
   Content-Length: [omitted]


   F15 200 OK Music Source -> Bob

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS atlanta.example.com:5061
    ;branch=z9hG4bK74rf
    ;received=192.0.2.103
   From: Bob <sips:bob@biloxi.example.com>;tag=02134
   To: Music Source <sips:music@source.example.com>;tag=56323
   Call-ID: 4802029847@biloxi.example.com
   Contact: <sips:music@source.example.com>;automaton
        ;+sip.byeless;+sip.rendering="no"
   CSeq: 2 BYE
   Content-Length: 0

   /* Normal media session between Alice and Bob is resumed. */




















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RFC 7088                      Music on Hold                February 2014


2.4.  Receiving Re-INVITE and UPDATE from the Remote UA

   While the call is on hold, the remote UA can send a request to modify
   the SDP or the feature parameters of its Contact header.  This can be
   done with either an INVITE or UPDATE method, both of which have much
   the same effect in regard to MOH.

   A common reason for a re-INVITE is when the remote UA desires to put
   the dialog on hold on its end.  And because of the need to support
   this case, an implementation must process INVITEs and UPDATEs during
   the on-hold state as described below.

   The executing UA handles these requests by echoing requests and
   responses: an incoming request from the remote UA causes the
   executing UA to send a similar request to the MOH source, and an
   incoming response from the MOH source causes the executing UA to send
   a similar response to the remote UA.  In all cases, SDP offers or
   answers that are received are added as bodies to the stimulated
   request or response to the other UA.

   The passed-through SDP will usually need its "o=" line modified.  The
   directionality attributes may need to be restricted by changing
   "active" to "recvonly" and "sendonly" to "inactive", as the executing
   UA will not render media from the remote UA.  (If all passed-through
   directionality attributes are "inactive", the optimization described
   in Section 2.10 may be applied.)  In regard to payload type numbers,
   since the mapping has already been established within the MOH dialog,
   "a=rtpmap" lines need not be added.

2.5.  Receiving INVITE with Replaces

   The executing UA must be prepared to receive an INVITE request with a
   Replaces header that specifies the dialog with the remote UA.  If the
   executing UA wants to create this new dialog in the on-hold state, it
   creates a new dialog with the MOH source to obtain MOH.  The
   executing UA negotiates the SDP within the dialog created by the
   INVITE with Replaces by passing the offer through to the new MOH
   dialog (if the INVITE contains an offer) or by creating the new MOH
   dialog with an offerless INVITE (if the INVITE does not contain an
   offer).











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   Continuing the example of Section 2.3, the executing UA receives an
   INVITE with Replaces that contains an offer:

   Alice             Bob       Music Source          Carol

   (For example, Alice has called Carol and initiates an attended
   transfer by sending a REFER to Carol, causing Carol to send an
   INVITE with Replaces to Bob.)

   Bob receives INVITE with Replaces from Carol:

     |                |              |                 |
     |                |              | INVITE/Replaces |
     |                |              | From: Carol     |
     |                |              | To: Bob         |
     |                |              | (SDP offer)     |
     |                |<-------------------------------|
     |                | INVITE       |                 |
     |                | From: Bob    |                 |
     |                | To: Music Source               |
     |                | (SDP offer,  |                 |
     |                |  rev. hold)  |                 |
     |                |------------->|                 |
     |                | 200 OK       |                 |
     |                | From: Bob    |                 |
     |                | To: Music Source               |
     |                | (SDP answer, |                 |
     |                |  hold)       |                 |
     |                |<-------------|                 |
     |                | ACK          |                 |
     |                | From: Bob    |                 |
     |                | To: Music Source               |
     |                |------------->|                 |
     |                |              | 200 OK          |
     |                |              | From: Carol     |
     |                |              | To: Bob         |
     |                |              | (SDP answer,    |
     |                |              |  hold)          |
     |                |------------------------------->|
     |                |              | ACK             |
     |                |              | From: Carol     |
     |                |              | To: Bob         |
     |                |<-------------------------------|
     |                |              | Music-on-hold RTP
     |                |              |================>|
     |                |              |                 |





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   Bob terminates the previous dialog with Alice:

     |                |              |                 |
     | BYE            |              |                 |
     | From: Bob      |              |                 |
     | To: Alice      |              |                 |
     |<---------------|              |                 |
     | 200 OK         |              |                 |
     | From: Bob      |              |                 |
     | To: Alice      |              |                 |
     |--------------->|              |                 |
     |                |              |                 |

   Bob terminates the MOH dialog for the dialog with Alice:

     |                |              |                 |
     |                | BYE          |                 |
     |                | From: Bob    |                 |
     |                | To: Music Source               |
     |                |------------->|                 |
     |                | 200 OK       |                 |
     |                | From: Music Source             |
     |                | To: Bob      |                 |
     |                |<-------------|                 |
     |                |              |                 |

   The new session continues on hold, between Bob and Carol.

2.6.  Receiving REFER from the Remote UA

   The executing UA must be prepared to receive a REFER request within
   the dialog with the remote UA.  The SDP within the dialog created by
   the REFER is negotiated by sending an offerless INVITE (or offerless
   re-INVITE) to the MOH source to obtain an offer and then using that
   offer in the INVITE to the refer target.

   Similar processing is used for an out-of-dialog REFER whose Target-
   Dialog header refers to the dialog with the remote UA.

   Continuing the example of Section 2.3, the executing UA receives an
   INVITE with Replaces that contains an offer:










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   Alice             Bob       Music Source          Carol

   (For example, Alice initiates an unattended transfer of the call to
   Carol by sending a REFER to Bob.)

   Bob receives REFER from Alice:

     |                |              |                 |
     | REFER          |              |                 |
     | From: Bob      |              |                 |
     | To: Alice      |              |                 |
     | Refer-To: Carol|              |                 |
     |--------------->|              |                 |
     |                | re-INVITE    |                 |
     |                | From: Bob    |                 |
     |                | To: Music Source               |
     |                | (no SDP)     |                 |
     |                |------------->|                 |
     |                | 200 OK       |                 |
     |                | From: Bob    |                 |
     |                | To: Music Source               |
     |                | (SDP offer,  |                 |
     |                |  hold)       |                 |
     |                |<-------------|                 |
     |                |              | INVITE          |
     |                |              | From: Bob       |
     |                |              | To: Carol       |
     |                |              | (SDP offer,     |
     |                |              |  hold)          |
     |                |------------------------------->|
     |                |              | 200 OK          |
     |                |              | From: Bob       |
     |                |              | To: Carol       |
     |                |              | (SDP answer,    |
     |                |              |  rev. hold)     |
     |                |------------------------------->|
     |                | ACK          |                 |
     |                | From: Bob    |                 |
     |                | To: Music Source               |
     |                | (SDP answer, |                 |
     |                |  rev. hold)  |                 |
     |                |------------->|                 |
     |                |              | ACK             |
     |                |              | From: Bob       |
     |                |              | To: Carol       |
     |                |------------------------------->|





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     |                |              | Music-on-hold RTP
     |                |              |================>|
     |                |              |                 |

   Bob terminates the previous dialog with Alice:

     |                |              |                 |
     | BYE            |              |                 |
     | From: Bob      |              |                 |
     | To: Alice      |              |                 |
     |<---------------|              |                 |
     | 200 OK         |              |                 |
     | From: Bob      |              |                 |
     | To: Alice      |              |                 |
     |--------------->|              |                 |
     |                |              |                 |

2.7.  Receiving Re-INVITE and UPDATE from the Music-on-Hold Source

   It is possible for the MOH source to send a re-INVITE or UPDATE
   request, and the executing UA can support doing so in similar manner
   as requests from the remote UA.  However, if the MOH source is within
   the same administrative domain as the executing UA, the executing UA
   may have knowledge that the MOH source will not (or need not) make
   such requests and so can respond to any such request with a failure
   response, avoiding the need to pass the request through.  The 403
   (Forbidden) response is suitable for this purpose because [RFC3261]
   specifies that this response indicates "the request SHOULD NOT be
   repeated".

   However, in an environment in which Interactive Connectivity
   Establishment (ICE) [RFC5245] is supported, the MOH source may need
   to send requests as part of ICE negotiation with the remote UA.
   Hence, in environments that support ICE, the executing UA must be
   able to pass through requests from the MOH source as well as requests
   from the remote UA.

   Again, as SDP is passed through, its "o=" line will need to be
   modified.  In some cases, the directionality attributes will need to
   be restricted.











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2.8.  Handling Payload Type Numbers

2.8.1.  Analysis

   In this technique, the MOH source generates an SDP answer that the
   executing UA presents to the remote UA as an answer within the
   original dialog.  In basic functionality, this presents no problem,
   because [RFC3264], Section 6.1 (at the very end) specifies that the
   payload type numbers used in either direction of RTP are the ones
   specified in the SDP sent by the recipient of the RTP.  Thus, the MOH
   source will send RTP to the remote UA using the payload type numbers
   specified in the offer SDP it received (ultimately) from the remote
   UA.

   But strict compliance to [RFC3264], Section 8.3.2 requires that
   payload type numbers used in SDP may only duplicate the payload type
   numbers used in any previous SDP sent in the same direction if the
   payload type numbers represent the same media format (codec) as they
   did previously.  However, the MOH source has no knowledge of the
   payload type numbers previously used in the original dialog, and it
   may accidentally specify a different media format for a previously
   used payload type number in its answer (or in a subsequently
   generated INVITE or UPDATE).  This would cause no problem with media
   decoding, as it cannot send any format that was not in the remote
   UA's offer, but it would violate [RFC3264].

   Strictly speaking, it is impossible to avoid this problem because the
   generator of a first answer in its dialog can choose the payload
   numbers independently of the payload numbers in the offer, and the
   MOH server believes that its answer is first in the dialog.  Thus,
   the only absolute solution is to have the executing UA rewrite the
   SDP that passes through it to reassign payload type numbers, which
   would also require it to rewrite the payload type numbers in the RTP
   packets -- a very undesirable solution.

   The difficulty solving this problem (and similar problems in other
   situations) argues that strict adherence should not be required to
   the rule that payload type numbers not be reused for different
   codecs.

   If an implementation of this technique were to interact with a remote
   UA that requires strict compliance to [RFC3264], the remote UA might
   reject the SDP provided by the MOH server.  (In Section 2.3, this SDP
   is in message F10.)  As a result, the MOH session will not be
   established, and the call will remain in its initial state.
   Implementors that wish to avoid this situation need to implement the
   solution in Section 2.8.2.




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2.8.2.  Solution to the Problem

   We can construct a technique that will strictly adhere to the payload
   type rule by exploiting a SHOULD-level requirement in [RFC3264],
   Section 6.1: "In the case of RTP, if a particular codec was
   referenced with a specific payload type number in the offer, that
   same payload type number SHOULD be used for that codec in the
   answer".  Or rather, we exploit the "implied requirement" that if a
   specific payload number in the offer is used for a particular codec,
   then the answer should not use that payload number for a different
   codec.  If the MOH source obeys this restriction, the executing UA
   can modify the offer SDP to "reserve" all payload type numbers that
   have ever been offered by the executing UA to prevent the MOH source
   from using them for different media formats.

   When the executing UA is composing the INVITE to the MOH source, it
   compiles a list of all the (dynamically assigned) payload type
   numbers and associated media formats that have been used by it (or by
   MOH sources on its behalf) in the original dialog.  (The executing UA
   must maintain a list of all previously used payload type numbers
   anyway, in order to comply with [RFC3264].)

   Any payload type number that is present in the offer but has been
   used previously by the executing UA in the original dialog for a
   different media format is rewritten to describe a dummy media format.
   (One dummy media format name can be used for many payload type
   numbers as multiple payload type numbers can refer to the same media
   format.)  A payload type number is added to describe the deleted
   media format, the number being either previously unused or previously
   used by the executing UA for that media format.

   Any further payload type numbers that have been used by the executing
   UA in the original dialog but that are not mapped to a media format
   in the current offer are then mapped to a dummy media format.

   The result is that the modified offer SDP:

   1.  offers the same set of media formats (ignoring dummies) as the
       original offer SDP (though possibly with different payload type
       numbers),

   2.  associates every payload type number either with a dummy media
       format or with the media format that the executing UA has
       previously used it for, and

   3.  provides a (real or dummy) media format for every payload type
       number that the executing UA has previously used.




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   These properties are sufficient to force an MOH server that obeys the
   implied requirement to generate an answer that is a correct answer to
   the original offer and is also compatible with previous SDP from the
   executing UA.

   Note that any re-INVITEs from the remote UA that the executing UA
   passes through to the MOH server require similar modification, as
   payload type numbers that the MOH server receives in past offers are
   not absolutely reserved against its use (as they have not been sent
   in SDP by the MOH server) nor is there a SHOULD-level proscription
   against using them in the current answer (as they do not appear in
   the current offer).

   This should provide an adequate solution to the problems with payload
   type numbers, as it will fail only if (1) the remote UA is particular
   that other UAs follow the rule about not redefining payload type
   numbers, and (2) the MOH server does not follow the implied
   requirement of [RFC3264], Section 6.1.

2.8.3.  Example of the Solution

   Let us show how this process works by modifying the example of
   Section 2.3 with this specific assignment of supported codecs:

      Alice supports formats X and Y.

      Bob supports formats X and Z.

      Music Source supports formats Y and Z.

   In this case, the SDP exchanges are:

      F1 offers X and Y, F3 answers X and Z.  (Only X can be used.)

      F6 offers X and Y, but F7 offers X, Y, and a place-holder to block
      use of type 92.

      F8/F10 answers Y.













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   The messages that are changed from Section 2.3 are:

    F1 INVITE Alice -> Bob

    INVITE sips:bob@biloxi.example.com SIP/2.0
    Via: SIP/2.0/TLS atlanta.example.com:5061
     ;branch=z9hG4bK74bf9
    Max-Forwards: 70
    From: Alice <sips:alice@atlanta.example.com>;tag=1234567
    To: Bob <sips:bob@biloxi.example.com>
    Call-ID: 12345600@atlanta.example.com
    CSeq: 1 INVITE
    Contact: <sips:a8342043f@atlanta.example.com;gr>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces, gruu
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
    s=
    c=IN IP4 atlanta.example.com
    t=0 0
    m=audio 49170 RTP/AVP 90 91
    a=rtpmap:90 X/8000
    a=rtpmap:91 Y/8000


    F3 200 OK Bob -> Alice

    SIP/2.0 200 OK
    Via: SIP/2.0/TLS atlanta.example.com:5061
     ;branch=z9hG4bK74bf9
     ;received=192.0.2.103
    From: Alice <sips:alice@atlanta.example.com>;tag=1234567
    To: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 1 INVITE
    Contact: <sips:bob@biloxi.example.com>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces
    Content-Type: application/sdp
    Content-Length: [omitted]








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    v=0
    o=bob 2890844527 2890844527 IN IP4 biloxi.example.com
    s=
    c=IN IP4 biloxi.example.com
    t=0 0
    m=audio 3456 RTP/AVP 90 92
    a=rtpmap:90 X/8000
    a=rtpmap:92 Z/8000


    F6 200 OK Alice -> Bob

    SIP/2.0 200 OK
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bK874bk
     ;received=192.0.2.105
    To: Alice <sips:alice@atlanta.example.com>;tag=1234567
    From: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 712 INVITE
    Contact: <sips:a8342043f@atlanta.example.com;gr>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces, gruu
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
    s=
    c=IN IP4 atlanta.example.com
    t=0 0
    m=audio 49170 RTP/AVP 90 91
    a=rtpmap:90 X/8000
    a=rtpmap:91 Y/8000
    a=active
















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    F7 INVITE Bob -> Music Source

    INVITE sips:music@source.example.com SIP/2.0
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bKnashds9
    Max-Forwards: 70
    From: Bob <sips:bob@biloxi.example.com>;tag=02134
    To: Music Source <sips:music@source.example.com>
    Call-ID: 4802029847@biloxi.example.com
    CSeq: 1 INVITE
    Contact: <sips:bob@biloxi.example.com>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces, gruu
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=bob 2890844534 2890844534 IN IP4 atlanta.example.com
    s=
    c=IN IP4 atlanta.example.com
    t=0 0
    m=audio 49170 RTP/AVP 90 91 92
    a=rtpmap:90 X/8000
    a=rtpmap:91 Y/8000
    a=rtpmap:92 x-reserved/8000
    a=recvonly


    F8 200 OK Music Source -> Bob

    SIP/2.0 200 OK
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bKnashds9
     ;received=192.0.2.105
    From: Bob <sips:bob@biloxi.example.com>;tag=02134
    To: Music Source <sips:music@source.example.com>;tag=56323
    Call-ID: 4802029847@biloxi.example.com
    Contact: <sips:music@source.example.com>;automaton
         ;+sip.byeless;+sip.rendering="no"
    CSeq: 1 INVITE
    Content-Length: [omitted]










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    v=0
    o=MusicSource 2890844576 2890844576 IN IP4 source.example.com
    s=
    c=IN IP4 source.example.com
    t=0 0
    m=audio 49170 RTP/AVP 91
    a=rtpmap:91 Y/8000
    a=sendonly

2.9.  Dialog/Session Timers

   The executing UA may discover that either the remote UA or the MOH
   source wishes to use dialog/session liveness timers [RFC4028].  Since
   the timers verify the liveness of dialogs, not sessions (despite the
   terminology of [RFC4028]), the executing UA can support the timers on
   each dialog (to the remote UA and to the MOH source) independently.
   (If the executing UA becomes obliged to initiate a refresh
   transaction, it must send an offerless UPDATE or re-INVITE, as if it
   sends an offer, the remote element has the opportunity to provide an
   answer that is different from its previous SDP, which could not
   easily be conveyed to the other remote element.)

2.10.  When the Media Stream Directionality is "inactive"

   The directionality of the media stream in the SDP offer in an INVITE
   or re-INVITE to the music source can be "inactive" if the SDP offer
   from the remote UA was "sendonly" or "inactive".  Generally, this
   happens when the remote UA also has put the call on hold and provided
   a directionality of "sendonly".  In this situation, the executing UA
   can omit establishing the dialog with the music source (or can
   terminate the existing dialog with the music source).

   If the executing UA uses this optimization, it creates the SDP answer
   itself, with directionality "inactive" and using its own RTP/RTCP
   ports, and returns that answer to the remote UA.

   The executing UA must be prepared for the remote UA to send a
   re-INVITE with directionality "active" or "recvonly", in which case
   the executing UA must initiate a dialog with the music source, as
   described above.

2.11.  Multiple Media Streams

   There may be multiple media streams (multiple "m=" lines) in any of
   the SDPs involved in the dialogs.  As the SDPs are manipulated, each
   media description (each starting with an "m=" line) is manipulated as
   described above for a single media stream, largely independently of
   the manipulation of the other media streams.  But there are some



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   elaborations that the executing UA may implement to achieve specific
   effects.

   If the executing UA desires to present only certain media types as
   on-hold media, when passing the offer SDP through, it can reject any
   particular media streams by setting the port number in the "m=" line
   to zero [RFC3264].  This ensures that the answer SDP will also have a
   rejection for that "m=" line.

   If the executing UA wishes to provide its own on-hold media for a
   particular "m=" line, it can do so by providing the answer
   information for that "m=" line.  The executing UA may decide to do
   this when the offer SDP is received (by modifying the "m=" line to
   rejected state when sending it to the music source) or upon receiving
   the answer from the music source and discovering that the "m=" line
   has been rejected.

   The executing UA may not want to pass a rejected "m=" line from the
   music source to the remote UA (when the remote UA provided a non-
   rejected "m=" line) and may instead provide an answer with
   directionality "inactive" (and specifying its own RTP/RTCP ports).

3.  Advantages

   This technique for providing music on hold has advantages over other
   methods now in use, including:

   1.  The original dialog is not transferred to another UA, so the
       "remote endpoint URI" displayed by the remote endpoint's user
       interface and dialog event package [RFC4235] does not change
       during the call, as contrasted to the method in [RFC5359],
       Section 2.3.  This URI is usually displayed to the user as the
       name and number of the other party on the call, and it is
       desirable for it not to change to that of the MOH server.

   2.  Compared to [RFC5359], this method does not require use of an
       out-of-dialog REFER, which is not otherwise used much in SIP.
       Out-of-dialog REFERs may not be routed correctly, since neither
       the From nor Contact URI of the original dialog may route
       correctly to the remote UA.  Also, out-of-dialog requests to UA
       URIs may not be handled correctly by authorization mechanisms.

   3.  The music-on-hold media are sent directly from the music-on-hold
       source to the remote UA, rather than being relayed through the
       executing UA.  This reduces the computational load on the
       executing UA and can reduce the load on the network (by
       eliminating "hairpinning" of the media through the link serving
       the executing UA).



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   4.  The remote UA sees, in the incoming SDP, the address/port that
       the MOH source will send MOH media from (assuming that the MOH
       source follows the convention of sending its media from its
       advertised media-listening address/port).  Thus, the remote UA
       will render the MOH media even if it is filtering incoming media
       based on originating address as a media security measure.

   5.  The technique requires relatively simple manipulation of SDP; in
       particular, (1) it does not require a SIP element to modify
       unrelated SDP to be acceptable to be sent within an already
       established sequence of SDP (a problem with [SIP-SERV-EX],
       Section 2.3), and (2) it does not require converting an SDP
       answer into an SDP offer (which was a problem with the initial
       draft version of this document, as well as with [SIP-SERV-EX]).

4.  Caveats

4.1.  Offering All Available Media Formats

   Unnecessary failures can happen if SDP offerers do not always offer
   all media formats that they support.  Doing so is considered best
   practice ([RFC6337], Sections 5.1 and 5.3), but some SIP elements
   offer only formats that have already been in use in the dialog.

   An example of how omitting media formats in an offer can lead to
   failure is as follows.  Suppose that the UAs in Section 2.3 each
   support the following media formats:

      Alice supports formats X and Y.

      Bob supports formats X and Z.

      Music Source supports formats Y and Z.

   In this case, the SDP exchanges are:

   1.  Alice calls Bob:
       Alice offers X and Y (message F1).
       Bob answers X (F3).

   2.  Bob puts Alice on hold:
       Alice (via Bob) offers X and Y (F6 and F7).
       Music Source (via Bob) answers Y (F8 and F10).

   3.  Bob takes Alice off hold:
       Bob offers X and Z (F11).
       Alice answers X (F12).




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   Note that in exchange 2, if Alice assumes that because only format X
   is currently in use that she should offer only X, the exchange fails.
   In exchange 3, Bob offers formats X and Z, even though neither is in
   use at the time (because Bob is not involved in the media streams).

4.2.  Handling Re-INVITES in a B2BUA

   Many UAs provide MOH in the interval during which it is processing a
   blind transfer, between receiving the REFER and receiving the final
   response to the stimulated INVITE.  This process involves switching
   the user's interface between three media sources: (1) the session of
   the original dialog, (2) the session with the MOH server, and (3) the
   session of the new dialog.  It also involves a number of race
   conditions that must be handled correctly.  If the UA is a back-to-
   back user agent (B2BUA) whose "other side" is maintaining a single
   dialog with another UA, each switching of media sources potentially
   causes a re-INVITE transaction within the other-side dialog.  Since
   re-INVITEs take time and must be sequenced correctly ([RFC3261],
   Section 14), such a B2BUA must allow the events on each side to be
   non-synchronous and must coordinate them correctly.  Failing to do so
   will lead to "glare" errors (491 or 500), leaving the other-side UA
   not rendering the correct session.

5.  Security Considerations

5.1.  Network Security

   Some mechanism outside the scope of this document must inform the
   executing UA of the MOH server that it should use.  Care must be
   exercised in selecting the MOH server, because signaling information
   that is part of the original dialog will be transmitted along the
   path from the executing UA to the server.  If the path between the
   executing UA and the server is not entirely contained within every
   network domain that contains the executing UA, the signaling between
   the UA and the server may be protected by different network security
   than is applied to the original dialog.

   Care must also be exercised because media information that is part of
   the original dialog will be transmitted along the path between the
   remote UA and the server.  If the path between the remote UA and the
   server does not pass through the same network domains as the path
   between the remote UA and the executing UA, the media between the UA
   and the server may be protected by different network security than is
   applied to the original dialog.







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   These requirements may be satisfied by selecting an MOH server that
   is in the same administrative and network domain as the executing UA
   and whose path to all external addresses is the same as the UA's path
   to those addresses.

5.2.  SIP (Signaling) Security

   The executing UA and the MOH server will usually be within the same
   administrative domain, and the SIP signaling path between them will
   lie entirely within that domain.  In this case, the administrator of
   the domain should configure the UA and server to apply to the dialog
   between them a level of security that is appropriate for the
   administrative domain.

   If the executing UA and the MOH server are not within the same
   administrative domain, the SIP signaling between them should be at
   least as secure as the SIP signaling between the executing UA and the
   remote UA.  Thus, the MOH server should support all of the SIP
   security facilities that are supported by the executing UA, and the
   executing UA should use in its dialog with the MOH server all SIP
   security facilities that are used in its dialog with the remote UA.

5.3.  RTP (Media) Security

   The RTP for the MOH media will pass directly between the MOH server
   and the remote UA and thus may pass outside the administrative domain
   of the executing UA.  While it is uncommon for the contents of the
   MOH media to be sensitive (and the remote UA will not usually be
   generating RTP when it is on hold), the MOH RTP should be at least as
   secure as the RTP between the executing UA and the remote UA.  In
   order to make this possible, the MOH server should support all of the
   RTP security facilities that are supported by the executing UA.

   It is possible that the remote UA and the MOH server support an RTP
   security facility that the executing UA does not support and that it
   is desirable to use this facility for the MOH RTP.  To enable doing
   so, the executing UA should pass the SDP between the remote UA and
   the MOH server completely, not omitting elements that it does not
   understand.

5.4.  Media Filtering

   Some UAs filter incoming RTP based on the address of origin as a
   media security measure, refusing to render the contents of RTP
   packets that originate from an address that is not shown in the
   remote SDP as an RTP destination address.  The remote UA in the
   original dialog may use this form of media filtering, and if the
   executing UA does not update the SDP to inform the remote UA of the



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   source address of the MOH media, the remote UA may not render the MOH
   media.  Note that the executing UA has no means for detecting that
   the remote UA uses media filtering, so the executing UA must assume
   that any remote UA uses media filtering.

   The technique described in this document ensures that any UA that
   should render MOH media will be informed of the source address of the
   media via the SDP that it receives.  This allows such UAs to filter
   media without interfering with MOH operation.

6.  Acknowledgments

   The original version of this proposal was derived from Section 2.3 of
   [SIP-SERV-EX] and the similar implementation of MOH in the snom UA.
   Significant improvements to the sequence of operations, allowing
   improvements to the SDP handling, were suggested by Venkatesh
   [VENKATESH].

   John Elwell [ELWELL] pointed out the need for the executing UA to
   pass through re-INVITEs/UPDATEs in order to allow ICE negotiation,
   suggested mentioning the role of RTCP listening ports, suggested the
   possibility of omitting the dialog to the music source if the
   directionality would be "inactive", and pointed out that if there are
   multiple media streams, the executing UA may want to select which
   streams receive MOH.

   Paul Kyzivat [KYZIVAT-1] [KYZIVAT-2] pointed out the difficulties
   regarding reuse of payload type numbers and considerations that could
   be used to avoid those difficulties, leading to the writing of
   Section 2.8.

   Paul Kyzivat suggested adding Section 4.1 showing why offerers should
   always include all supported formats.

   M. Ranganathan pointed out the difficulties experienced by a B2BUA
   (Section 4.2) due to the multiple changes of media source.

   Section 4.1 was significantly clarified based on advice from Attila
   Sipos [SIPOS].

   The need to discuss dialog/session timers (Section 2.9) was pointed
   out by Rifaat Shekh-Yusef [SHEKH-YUSEF].

   Robert Sparks clarified the purpose of the "Best Current Practice"
   status, leading to revising the intended status of this document to
   "Informational".





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   In his SecDir review, Stephen Kent pointed out that the Security
   Considerations should discuss the use of SIP and SDP security
   features by the MOH server.

   Numerous improvements to the text were due to reviewers, including
   Rifaat Shekh-Yusef and Richard Barnes.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264, June
              2002.

   [RFC4028]  Donovan, S. and J. Rosenberg, "Session Timers in the
              Session Initiation Protocol (SIP)", RFC 4028, April 2005.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

7.2.  Informative References

   [RFC4235]  Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
              Initiated Dialog Event Package for the Session Initiation
              Protocol (SIP)", RFC 4235, November 2005.

   [RFC4961]  Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)",
              BCP 131, RFC 4961, July 2007.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April
              2010.

   [RFC5359]  Johnston, A., Sparks, R., Cunningham, C., Donovan, S., and
              K. Summers, "Session Initiation Protocol Service
              Examples", BCP 144, RFC 5359, October 2008.





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   [RFC6337]  Okumura, S., Sawada, T., and P. Kyzivat, "Session
              Initiation Protocol (SIP) Usage of the Offer/Answer
              Model", RFC 6337, August 2011.

   [ELWELL]   Elwell, J., "Subject: [Sipping] RE: I-D Action:draft-
              worley-service-example-00.txt", message to the IETF
              Sipping mailing list, November 2007,
              <http://www1.ietf.org/mail-
              archive/web/sipping/current/msg14678.html>.

   [KYZIVAT-1]
              Kyzivat, P., "Subject: Re: [Sipping] I-D ACTION:draft-
              ietf-sipping-service-examples-11.txt", message to the IETF
              Sipping mailing list, October 2006, <http://www1.ietf.org/
              mail-archive/web/sipping/current/msg12181.html>.

   [KYZIVAT-2]
              Kyzivat, P., "Subject: [Sip-implementors] draft-worley-
              service-example-02", message to the sip-implementors
              mailing list, September 2008,
              <http://lists.cs.columbia.edu/pipermail/sip-implementors/
              2008-September/020394.html>.

   [SHEKH-YUSEF]
              Shekh-Yusef, R., "Subject: [sipcore] draft-worley-service-
              example-03", message to the IETF Sipcore mailing list,
              July 2009, <http://www.ietf.org/mail-archive/web/sipcore/
              current/msg00580.html>.

   [SIPOS]    Sipos, A., "Subject: [Sip-implementors] draft-worley-
              service-example-02", message to the sip-implementors
              mailing list, March 2009, <http://lists.cs.columbia.edu/
              pipermail/sip-implementors/2009-March/021970.html>.

   [SIP-SERV-EX]
              Johnston, A., Sparks, R., Cunningham, C., Donovan, S., and
              K. Summers, "Session Initiation Protocol Service
              Examples", Work in Progress, October 2006.

   [VENKATESH]
              Venkatesh, "Subject: Re: [Sipping] I-D ACTION:draft-
              ietf-sipping-service-examples-11.txt", message to the IETF
              Sipping mailing list, October 2006, <http://www1.ietf.org/
              mail-archive/web/sipping/current/msg12180.html>.







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Author's Address

   Dale R. Worley
   Ariadne Internet Services, Inc.
   738 Main St.
   Waltham, MA  02451
   US

   Phone: +1 781 647 9199
   EMail: worley@ariadne.com









































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