Session Initiation Protocol Service Example -- Music on Hold
RFC 7088

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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RFC 7088 Music on Hold February 2014
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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RFC 7088 Music on Hold February 2014
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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RFC 7088 Music on Hold February 2014
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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RFC 7088 Music on Hold February 2014
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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RFC 7088 Music on Hold February 2014
 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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RFC 7088 Music on Hold February 2014
 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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RFC 7088 Music on Hold February 2014
 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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RFC 7088 Music on Hold February 2014
 /* 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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RFC 7088 Music on Hold February 2014
 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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RFC 7088 Music on Hold February 2014
 /* 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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RFC 7088 Music on Hold February 2014
 /* 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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 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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RFC 7088 Music on Hold February 2014
 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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RFC 7088 Music on Hold February 2014
 [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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RFC 7088 Music on Hold February 2014
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
Worley Informational [Page 36]