SIP D. Worley
Internet-Draft Nortel Networks
Intended status: BCP March 5, 2009
Expires: September 6, 2009
Session Initiation Protocol Service Example -- Music on Hold
draft-worley-service-example-03
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Abstract
The "music on hold" feature is one of the most desired features of
telephone systems in the business environment. "Music on hold" is
where, 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 compliant with the
standards. The implementation of music-on-hold described in this
document is fully effective and standards-compliant, but is simpler
than the methods previously documented.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Technique . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Placing a Call on Hold and Providing an External Media
Stream . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Taking a Call off Hold and Terminating the External
Media Stream . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Example Message Flow . . . . . . . . . . . . . . . . . . . 6
2.4. Re-INVITE and UPDATE from the Remote UA . . . . . . . . . 14
2.5. INVITE with Replaces . . . . . . . . . . . . . . . . . . . 15
2.6. Re-INVITE and UPDATE from the Music-On-Hold Source . . . . 15
2.7. Payload Type Numbers . . . . . . . . . . . . . . . . . . . 15
3. Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4. The Importance of Offering All Available Media Formats . . . . 22
5. Security Considerations . . . . . . . . . . . . . . . . . . . 23
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
7. Revision History . . . . . . . . . . . . . . . . . . . . . . . 25
7.1. Changes from draft-worley-service-example-00 to
draft-worley-service-example-01 . . . . . . . . . . . . . 25
7.2. Changes from draft-worley-service-example-01 to
draft-worley-service-example-02 . . . . . . . . . . . . . 25
7.3. Changes from draft-worley-service-example-02 to
draft-worley-service-example-03 . . . . . . . . . . . . . 25
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
8.1. Normative References . . . . . . . . . . . . . . . . . . . 26
8.2. Informative References . . . . . . . . . . . . . . . . . . 26
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 28
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1. Introduction
Within SIP[sip]-based systems, 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": The
music-on-hold feature is where, 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 compliant with the standards. The
purpose of this document is to describe a method that is reasonably
simple yet fully effective and standards-compliant.
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2. Technique
The essence of the technique is that when the executing UA (the
user's UA) performs a re-INVITE of the remote UA to establish the
hold state, it provides no SDP[sdp]
offer[offer-answer][offer-answer-bis], 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.
2.1. Placing a Call on Hold and Providing 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. ([ref-dialog-event] 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
perhaps "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 2.7]
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[offer-answer]
specify "sendonly" or "inactive" as the directionality for all
media streams. (Although this address/port should receive no
RTP, by convention UAs use their declared RTP listening ports as
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their RTP source ports as well. 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 as SPIT. This functionality requires the
SDP answer to contain the sending address/port in the c= line,
even though the MOH source does not receive RTP.)
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 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 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, and 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 re-established in the original dialog.
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2.3. Example Message Flow
This section shows a message flow which is an example of this
technique. The scenario is: 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.
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, |
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| | hold) |
| |<-------------|
| | ACK F9 |
| |------------->|
| | |
Bob's UA provides an SDP answer containing the address/port
of the 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.
/* Alice calls Bob. */
F1 INVITE Alice -> Bob
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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
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>
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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
/* 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
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/* 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
/* 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
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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
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>
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
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/* 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 the Music Source,
and thus contains the address/port from which the Music Source
will send media. */
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
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s=
c=IN IP4 biloxi.example.com
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
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
F14 BYE Bob -> Music Source
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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 -> Alice
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
CSeq: 2 BYE
Content-Length: 0
/* Normal media session between Alice and Bob is resumed */
2.4. 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 will be 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.
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The passed-through SDP will usually need its o= line modified. The
directionality attributes may need to be restricted. 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. INVITE with Replaces
The executing UA must be prepared to receive INVITE requests with a
Replaces headers that replaces the original dialog, and similarly it
must be prepared to receive REFER requests within the dialog. The
SDP within the new dialog is negotiated by being passed through to
the MOH source within a new dialog with the MOH source. The SDP
offer or answer can be passed to the MOH source with only
modification to the o= line and directionality attributes.
In some cases, the previous dialog with the MOH source can be reused,
but only if the executing UA presents the first offer within the new
dialog, as otherwise there is no way to force the RTP payload types
that have been used previously in the MOH dialog to be mapped to the
correct codecs in the new dialog.
2.6. Re-INVITE and UPDATE from the Music-On-Hold Source
It is possible for the MOH source to send an 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.
However, in an environment in which ICE[ice] is supported, the MOH
source may need to send requests as part of ICE negotiation[elwell]
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.
2.7. Payload Type Numbers
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 [offer-answer] (section 6.1, at the very end) specifies that
the payload type numbers used in either direction of RTP are the ones
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specified in the SDP sent by the recipient of the RTP.
But strict compliance to [offer-answer] (section 8.3.2) requires that
payload type numbers used in SDP may only duplicate the payload type
numbers used in any SDP used in the same direction in the dialog 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 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 [offer-answer].
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. But we can exploit a SHOULD-
level requirement in [offer-answer] (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." 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 which have been used by it (or by MOH sources on its behalf)
in the original dialog but which are not mapped to a media format in
the current offer SDP. (The executing UA must be maintaining a list
of all previously used payload type numbers anyway, in order to
comply with [offer-answer].) Then, for each of these payload type
numbers, it inserts session-level or media-level (as appropriate)
a=rtpmap lines specifying the payload type number and the media
format that it has been used for. Because of the reuse rule, the MOH
source SHOULD not propose those payload type numbers for any other
media format.
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
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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 re-defining payload type
numbers, and (2) the MOH server does not follow the SHOULD-level
requirement of [offer-answer] section 6.1.
Let us show how this process works by modifying the example
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 (which cannot be used)
F6 offers X and Y, but F7 offers X, Y, and a place-holder to block
type 92
F8/F10 answers Y
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=
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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]
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]
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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
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
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
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Contact: <sips:music@source.example.com>
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 91
a=rtpmap:91 Y/8000
a=sendonly
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3. Advantages
This technique for providing music-on-hold has advantages over other
methods now in use:
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[ref-dialog-event] does not
change during the call.[service-examples]
2. 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.
3. The remote UA sees, in the incoming SDP, the address/port that
the MOH source will send MOH media from, thus allowing it to
render the media, even if it is filtering incoming media based on
originating address as a SPIT preventative.
4. The technique requires relatively simple manipulation of SDP, and
in particular: (1) 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
[service-examples-11]), and (2) does not require converting an
SDP answer into an SDP offer (which was a problem with the -00
version of this document, as well as with [service-examples-11]).
5. It complies with the payload type number rules.[offer-answer]
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4. The Importance of Offering All Available Media Formats
Failures can happen if SDP offerers do not always offer all media
formats that they support. Doing so is considered best practice, but
some elements will 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 inSection 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:
F1 offers X and Y, F3 answers X
F6/F7 offers X and Y, F8/F10 answers Y
F11 offers X and Z, F12 answers X
Note that in exchange 2, if Alice assumes that because only format X
is 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).
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5. Security Considerations
Some UAs filter incoming media based on the address of origin in
order to avoid SPIT. 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
should allow such UAs to filter without interfering with MOH
operation.
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6. Acknowledgments
The original version of this proposal was derived from
[service-examples-11] 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.
Paul Kyzivat[kyzivat] pointed out the difficulties regarding re-use
of payload type numbers.
Paul Kyzivat suggested adding sectionSection 4 showing why offerers
should always include all supported formats.
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7. Revision History
7.1. Changes from draft-worley-service-example-00 to
draft-worley-service-example-01
Removed the original "Example Message Flow" and promoted the
"Alternative Example Message Flow" to replace it because of a number
of flaws that were found during the discussion of -00 on the SIPPING
mailing list.
Described the use of the sip.rendering feature parameter to indicate
on-hold status.
7.2. Changes from draft-worley-service-example-01 to
draft-worley-service-example-02
Added discussion of passing though re-INVITEs and UPDATEs.
Added discussion of payload type numbers.
Added Acknowledgments section.
7.3. Changes from draft-worley-service-example-02 to
draft-worley-service-example-03
Added sectionSection 4 showing the importance of the offerer always
including all supported media formats.
Updated references.
Revised handling of payload type numbers when passing offer to MOH
serverSection 2.7 based on observations by Paul Kyzivat.
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8. References
8.1. Normative References
[sip] 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.
[offer-answer]
Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with the Session Description Protocol (SDP)", RFC 3264,
June 2002.
[sdp] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
8.2. Informative References
[service-examples]
Johnston, A., Sparks, R., Cunningham, C., Donovan, S., and
K. Summers, "Session Initiation Protocol Service
Examples", RFC 5359, October 200.
[service-examples-11]
Johnston, A., Sparks, R., Cunningham, C., Donovan, S., and
K. Summers, "Session Initiation Protocol Service
Examples", I-D draft-ietf-sipping-service-examples-11,
October 2006.
[ref-dialog-event]
Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session Initiation
Protocol (SIP)", RFC 4235, November 2005.
[venkatesh]
Venkatesh, "Subject: Re: [Sipping] I-D
ACTION:draft-ietf-sipping-service-examples-11.txt", IETF
Sipping mailing list msg12180, October 2006.
[ice] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
I-D draft-ietf-mmusic-ice-19, October 2007.
[elwell] Elwell, J., "Subject: [Sipping] RE: I-D
Action:draft-worley-service-example-00.txt", IETF Sipping
mailing list msg14678, November 2007.
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[kyzivat] Kyzivat, P., "Subject: Re: [Sipping] I-D
ACTION:draft-ietf-sipping-service-examples-11.txt", IETF
Sipping mailing list msg12181, October 2006.
[offer-answer-bis]
Sawada, T. and P. Kyzivat, "SIP (Session Initiation
Protocol) Usage of the Offer/Answer Model",
I-D draft-ietf-sipping-sip-offeranswer-10, January 2009.
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Author's Address
Dale R. Worley
Nortel Networks Corp.
600 Technology Park Dr.
Billerica, MA 01821
US
Email: dworley@nortel.com
URI: http://www.nortel.com
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