Internet DRAFT - draft-tveretin-dispatch-remote
draft-tveretin-dispatch-remote
Network Working Group A. Tveretin
Internet-Draft April 10, 2017
Intended status: Standards Track
Expires: October 12, 2017
Remote Call Control and Call Pick-up in SIP
draft-tveretin-dispatch-remote-03.txt
Abstract
This memo defines a mechanism by which a SIP user agent could inspect
calls at another user agent, and control a call, including picking up
for itself.
Status of This Memo
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Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 2
2. Usage Scenario . . . . . . . . . . . . . . . . . . . . . . . 2
3. Implementation Basics . . . . . . . . . . . . . . . . . . . . 2
4. Terms Used In This Specification . . . . . . . . . . . . . . 3
5. Dialog Subscription (revised) . . . . . . . . . . . . . . . . 3
5.1. Event package name . . . . . . . . . . . . . . . . . . . 3
5.2. SUBSCRIBE Request Bodies . . . . . . . . . . . . . . . . 3
5.3. Subscription Duration . . . . . . . . . . . . . . . . . . 3
5.4. NOTIFY Request Bodies . . . . . . . . . . . . . . . . . . 3
5.5. Notifier processing of SUBSCRIBE Requests . . . . . . . . 3
5.6. Notifier generation of NOTIFY Requests . . . . . . . . . 3
5.7. Subscriber processing of NOTIFY Requests . . . . . . . . 4
5.8. Handling of Forked Requests . . . . . . . . . . . . . . . 4
5.9. Rate of Notifications . . . . . . . . . . . . . . . . . . 4
5.10. State Aggregation . . . . . . . . . . . . . . . . . . . . 4
6. New Methods . . . . . . . . . . . . . . . . . . . . . . . . . 4
6.1. ANSWER Method Definition . . . . . . . . . . . . . . . . 5
6.2. PICKUP Method Definition . . . . . . . . . . . . . . . . 6
6.3. REJECT Method Definition . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10. Normative References . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Comparison message flows with RFC 3891 . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Requirements notation
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. Usage Scenario
Alice calls Bob. Bob hears the call from B1 device (e.g. fixed).
However, Bob wants to answer (or reject) that call from another
device B2 (e.g. mobile).
For more precise definition of this service, see [H.450.5]
3. Implementation Basics
First, B2 MUST identify the call umambigously. The only way for this
is to use dialog-id [RFC3261]. Thus, B2 must ask B1 somehow for the
dialog-id in question. An [RFC4235] solution exists, but only some
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information will be supplied. Thus, an alternative is suggested: use
message/sip media type for dialog information.
Note: this does not affect existing implementations and
interoperability.
Any given UA at any given time may hold multiple dialogs. After the
affected dialog is chosen, B2 orders B1 to handle it (answer, reject,
pick up [forward to B2], or forward somewhere else.
4. Terms Used In This Specification
Affected dialog (AfD): a SIP dialog whose state changed by remote
call control; Controlling party (Ctg): an entity requesting call
monitoring, or issuing remote call control commands (Note: this
definition does not require to use SIP). Controlled party (Ctd): an
entity to which remote call control commands are directed. It is
also a party of Affected dialog.
5. Dialog Subscription (revised)
5.1. Event package name
This specification reuses the dialog event package, which is defined
in [RFC4235]
5.2. SUBSCRIBE Request Bodies
SUBSCRIBE request body is empty.
5.3. Subscription Duration
5.4. NOTIFY Request Bodies
NOTIFY request body comforming to this specification is a valid SIP
message (request or response). It has "message/sip" MIME type. This
specification also allows multipart bodies.
5.5. Notifier processing of SUBSCRIBE Requests
As per [RFC6665]. However, consider security implications.
5.6. Notifier generation of NOTIFY Requests
The Ctd will relay SIP messages, both incoming and outgoing, that
alter state of any dialog. It MUST NOT relay any messages (e.g.
MESSAGE, OPTIONS, ANSWER, PICKUP, REJECT requests and responses) that
do not imply any kind of dialog state change themselves. The Ctd MAY
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strip SIP header fields and SIP bodies that do not indicate state of
calls, including information hidden from Ctd for privacy reasons.
However, this memo does not suggest any.
Messages are packed unencrypted.
In addition, Ctd will add the 'direction' parameter to the Event
header field. The value is a token indicating the direction of the
call (not dialog). Currently, predefined values are: incoming,
outgoing, transit, external, internal, 3pcc, unknown, none. FFS:
should this form an IANA registry?
5.7. Subscriber processing of NOTIFY Requests
Cntrlg discriminates incoming and outgoing (from Ctd perspective)
messages by comparing known AoR with From-URI and To-URI. Then Ctg
uses this information to track the state of dialogs. It MAY also
alter call state by sending appropriate requests.
5.8. Handling of Forked Requests
See [RFC6665].
5.9. Rate of Notifications
Remote call control is done in near real time. So no trottling is
allowed. Nor it is allowed to suppress notifications, which may
bring devices out-of-sync.
5.10. State Aggregation
This specification covers state changes presumable triggered by user.
As such, they are assumed to be rare.
It is still possible to combine several messages (using multipart/
mixed) into a single NOTIFY request. In that case, they retain
relative order.
6. New Methods
This document introduces new methods, namely ANSWER, PICKUP, and
REJECT, into SIP. Existing methods considered unsuitable for the
task.
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6.1. ANSWER Method Definition
The ANSWER method has semantics of answering the call, or otherwise
establishing a dialog. Ctg provides instructions for this, via
request-body.
Structure: header field are those mandated by SIP (From, To, Call-Id,
Contact, Via)
To-tag: this MUST NOT be present, as the request is out-of-dialog.
Answer-Mode, Priv-Answer-Mode: as by [RFC5373]. "Auto" implies
automatic reception of faxes, files, or voice mail (or video mail)
etc. "Manual" implies using a loudspeaker for the call. This
distinction has less sense for faxes. Exactly one of them is
REQUIRED.
Replaces: header field is REQUIRED, it indicates the AFD.
Target-Dialog: SHOULD refer to an existing dialog between Ctg and
Ctd, if any.
Referred-By: OPTIONALLY reveals the Ctg to the caller. If present,
it is relayed in the 200 or equivalent response.
Subject: is not relayed, but could be used by Ctd for logging.
The request body is a URI. It is used by the Ctd to answer
anonymously or not. Thus, the body becomes To-URI of the response.
Detailed execution:
Early-dialog at UAS side is answered (200 response).
A subscription at subscriber side is renewed.
A subscription at notifier side is renewed. Note that the NOTIFY
request act as a "response" for the purpose of this section.
For established dialogs, this method fails.
Request MAY fail because of e.g. race condition. Ctd reports actual
state by exisiting subscriptions (or equivalent monitoring); Ctg MUST
NOT rely on responses to track outcome of call control. For this
reason, responses are defined to contain only minimum required
information, and in particular, no body.
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6.2. PICKUP Method Definition
The PICKUP method has semantics of forwarding the call to Ctg, or to
another party. Unlike forwarding proper, it is initiated by Ctg as
forwarded-to, rather than the forwarder. Ctd also relays any extra
information (Referred-By:, Contact:, Reason: header fields, or the
request body) to its peer.
Structure: header field are those mandated by SIP (From, To, Call-Id,
Contact, Via)
To-tag: this MUST NOT be present, as the request is out-of-dialog.
Replaces: header field is REQUIRED, it indicates the AFD.
Target-Dialog: SHOULD refer to an existing dialog between Ctg and
Ctd, if any.
Reason: SHOULD be present. This information is exposed to the
caller.
Referred-By: OPTIONALLY reveals the Ctg to the caller. If present,
it is relayed to the caller.
Refer-To: may reference the Ctg (this is the default), or the call
may be forwarded to other entity. This header field is OPTIONAL.
Subject: is not relayed, but could be used by Ctd for logging.
The request body is relayed just as the Referred-By: header field.
Detailed execution:
UAS forwards the affected early-dialog by sending 302 (or 3xx for
that reason).
UAC forwards the affected early-dialog by cancelling it (i.e. sending
the CANCEL request) and making another call.
An established dialog is transferred.
Similar behaviour is expected for subscriptions at subscriber side.
6.3. REJECT Method Definition
The REJECT method has semantics of tearing down a dialog, including
rejection of an incoming call (early dialog).
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Structure: header field are those mandated by SIP (From, To, Call-Id,
Contact, Via)
To-tag: this MUST NOT be present, as the request is out-of-dialog.
Replaces: header field is REQUIRED, it indicates the AFD.
Target-Dialog: SHOULD refer to an existing dialog between Ctg and
Ctd, if any.
Reason: SHOULD be present. This information is exposed to the
caller.
Referred-By: OPTIONALLY reveals the Ctg to the caller. If present,
it is relayed to the caller.
Subject: is not relayed, but could be used by Ctd for logging.
The request body is relayed just as the Referred-By: header field.
It MAY be used as a detailed error message.
Detailed execution:
Early-dialog at UAS side is rejected by a 4xx-5xx response, using the
reason code of the REJECT request.
Early-dialog at UAC side is cancelled with a CANCEL request, using
the reason code of the REJECT request.
An established dialog is terminated with a BYE request, using the
reason code of the REJECT request.
A subscription is terminated (by reducing time to to zero) by either
subscriber or notifier.
7. IANA Considerations
This memo introduces 3 new methods for SIP. Namely, ANSWER PICKUP
REJECT For details, please refer to this specification, RFC xxxx
8. Security Considerations
All information related to calls (including the fact of the call) is
sensitive. An appropriate authorization of Ctg by Ctd is REQUIRED.
In particular, Target-Dialog MUST NOT be used (nor expected to)
instead of actual authentication.
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This memo assumes that Ctg and Ctd belong to the same person or
entity. This, they can have any shared secret (even symmetric
cryptography key). In practice, end-to-end information flow may be
hindered by network, especially if interworking is involved. It is
RECOMMENDED that remote control is not used in doubtful cases.
Another issue is the implementation of the ANSWER method. It may be
unexpected even by authorized Ctg, e.g. receiving a fax or turning on
a loudspeaker. All behaviour MUST be documented well, including
warnings when needed.
9. Example
For scenario:
INVITE sip:bob@biloxi.com SIP/2.0
From: Alice <alice@atlanta.com>;tag=tag1
To: Bob <bob@biloxi.com>
Subject: Hello
Content-Type: application/sdp
Contact: <sip:192.168.2.101>
Call-Id: 15@atlanta.com
/*More header skipped*/
SDP...
Now consider Carol intercepting this call.
First, any further information is requested:
SUBSCRIBE bob@biloxi.com SIP/2.0
From: Carol <carol@cover.com>;tag=tag3
To: Bob <bob@biloxi.com>
Event: dialog
Accept-types: message/sip, application/sdp
Expire:
/*More header skipped*/
Bob responds, and sends a notification:
NOTIFY sip:192.168.2.301 SIP/2.0
From: Bob <bob@biloxi.com>;tag=tag4
To: Carol <carol@cover.com>;tag=tag3
Event: dialog
Content-Type: message/sip
/*more...*/
Now Carol chooses to answer this call. She sends a PICKUP:
PICKUP sip:192.168.2.201 SIP/2.0
From: Carol <carol@cover.com>;tag=tag83
To: Bob <bob@biloxi.com>
Refer-To: Carol@cover.com
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Referred-By: Carol@cover.com
Replaces: 15@atlanta.com;local-tag=tag2;remote-tag=tag1
Target-Dialog: ...;local-tag=tag3;remote-tag=tag4
Reason: SIP;cause=500;text=Picked-Up
Content-Length: 0
/*skipped*/
Now Bob responds to Alice:
SIP/2.0 302 Picked Up
From: Alice <alice@atlanta.com>;tag=tag1
To: Bob <bob@biloxi.com>;tag=tag2
History-Info: <sip:bob@biloxi.com?reason=sip%3bcause%3d500>;index=1
Contact: Carol@cover.com
Content-Length: 0
The success of this operation is also reported to by with 200 PICKUP
and NOTIFY messages.
10. Normative References
[H.450.5] ITU-T, "Call Park and Call Pickup Supplementary Services
for H.323.", 1999, <Rec. H.450.5>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
[RFC4235] Rosenberg, J., Schulzrinne, H., and R. Mahy, Ed., "An
INVITE-Initiated Dialog Event Package for the Session
Initiation Protocol (SIP)", RFC 4235,
DOI 10.17487/RFC4235, November 2005,
<http://www.rfc-editor.org/info/rfc4235>.
[RFC5373] Willis, D., Ed. and A. Allen, "Requesting Answering Modes
for the Session Initiation Protocol (SIP)", RFC 5373,
DOI 10.17487/RFC5373, November 2008,
<http://www.rfc-editor.org/info/rfc5373>.
[RFC6665] Roach, A., "SIP-Specific Event Notification", RFC 6665,
DOI 10.17487/RFC6665, July 2012,
<http://www.rfc-editor.org/info/rfc6665>.
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Appendix A. Comparison message flows with RFC 3891
RFC 3891 gives an impression that it is minimal message flow for a
call pick-up or equivalent. The fact is this RFC assumes the same
number of SIP messages.
Incoming call:
A->B: INVITE
B->A: 180
C->B: SUBSCRIBE
B->C: 200 SUBSCRIBE
B->C: NOTIFY
C->B: 200 NOTIFY
RFC 3891:
C->A: INVITE/Replaces
A->B: CANCEL
A->C: 200 INVITE
B->A: 200 CANCEL
B->A: 451 INVITE
A->B: ACK
C->A: ACK
B->C: NOTIFY
C->B: 200 NOTIFY
This memo:
C->B: PICKUP
B->C: 200 PICKUP
B->A: 302 INVITE
A->B: ACK
B->C: NOTIFY
C->B: 200 NOTIFY
A->C: INVITE/History-Info
C->A: 200 INVITE
A->C: ACK
An alternative, draft-, is almost the same as this RFC, except PICKUP
is substituted for REFER/Replaces.
Author's Address
Anton Tveretin
ul.Dzerzhinskogo, d. 13/1, kv.34
Surgut, HMAO-Yugra 628416
RU
Phone: +79224149328
Email: tveretinas@yandex.ru
URI: http://www.fit-rulez.narod.ru
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