sipping D. R. Worley
Internet-Draft Pingtel
Expires: August 30, 2006 February 26, 2006
Call Pickup Examples in SIP
draft-worley-sipping-pickup-01
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Abstract
This document walks through various examples and call flows for
implementing "call pickup" operations in SIP telephony. It focuses
on distributing as much processing as possible in the user agents in
accordance with the philosophy of end-point controlled call-control
(EPCC).
Various difficulties in implementing call pickup are discussed, and
further suggestions and discussion are solicited.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Basic call flow . . . . . . . . . . . . . . . . . . . . . . . 4
3. Detailed call flow . . . . . . . . . . . . . . . . . . . . . . 6
4. Call pickup with proxy . . . . . . . . . . . . . . . . . . . . 9
5. Call pickup with proxy and state agent . . . . . . . . . . . . 12
6. Call pickup with a pickup agent . . . . . . . . . . . . . . . 14
7. With improved pickup agent . . . . . . . . . . . . . . . . . . 16
8. With proxy and improved pickup agent . . . . . . . . . . . . . 18
9. Group call pickup . . . . . . . . . . . . . . . . . . . . . . 21
9.1. Picking up any call to any member of a group of
individuals . . . . . . . . . . . . . . . . . . . . . . . 21
9.2. Picking up a call routed to several individuals . . . . . 21
9.3. Picking up a call that is not routed to a specific
user agent . . . . . . . . . . . . . . . . . . . . . . . . 21
10. Call park . . . . . . . . . . . . . . . . . . . . . . . . . . 22
11. Security considerations . . . . . . . . . . . . . . . . . . . 23
12. Informative References . . . . . . . . . . . . . . . . . . . . 23
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 25
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1. Introduction
There are several different schemes for implementing call pickup.
The basic method is the one specified in the Sylantro "SIP-B"
specification, which despite its proprietary air, uses standard SIP
features in an end-point call control (EPCC) style [1]. All other
methods are variations on the same theme, usually by using an agent
process (in a proxy or communications server) to provide a feature
that the user agents are lacking. Almost all the software
development effort to support call pickup is in implementing these
agent processes, so we can easily see the trade-off between
development effort and which features must be provided by the user
agents.
Like call transfer, effecting call pickup requires some support from
the caller's end. These caller-end features will, therefore, soon
come to be considered necessary for any "quality" SIP implementation.
This document shows message flows for executing call pickup using
various configurations of SIP agents, including a proxy that
dispatches requests for AORs to their registered UAs, "call state
agents" that generate dialog events on behalf of UAs that cannot
generate them, and "pickup agents" that execute the call pickup
sequence on behalf of UAs that cannot do so themselves. It also
discusses various complications that can arise.
This document is not intended to advance toward standardization, but
rather to server as examples for implementors, and to start
discussion of problems in SIP that may arise when implementing call
pickup.
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2. Basic call flow
The SIP-B pickup mechanism is implemented in the user agents. Thus,
it requires a more sophisticated user agent to execute the pickup.
It also depends on the callee phone producing dialog events, or there
being a state agent doing so on its behalf.
The basic SIP-B pickup sequences is as follows. (Only principal
messages are shown.) Suppose the incoming call is to the callee
phone, extension 123, and the phone executing the pickup is extension
456:
Caller Callee Executing
123 456
| | |
| 1 INVITE | |
|---------------------->| |
| | |
| | | 2 Call Pickup
| | | on 123
| | |
| | 3 SUBSCRIBE |
| |<------------------|
| | |
| | 4 NOTIFY |
| |------------------>|
| | |
| 5 INVITE/Replaces | |
|<------------------------------------------|
| | |
| 6 CANCEL | |
|---------------------->| |
| | |
| 7 200 OK | |
|------------------------------------------>|
| | |
| | |
1. The caller (AOR Caller@example.com) sends an INVITE with URI
123@example.com to phone 123.
2. The user of phone 456 activates the call pickup feature for
extension 123.
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3. Phone 456 sends a SUBSCRIBE with URI 123@example.com to phone 123,
requesting "Event: dialog" and "Expires: 0".
4. Phone 123 sends one NOTIFY to phone 456 giving the status of its
current dialogs for AOR 123@example.com, which includes the early
dialog of INVITE 1 from Caller, and gives the "remote identity"
and "remote target" of the dialog (which are the From: and
Contact: of INVITE 1), one of which is "Caller@example.com".
5. Phone 456 sends INVITE 5 to Caller@example.com. It has a
Replaces: header specifying the dialog parameters sent in the
NOTIFY. The Replaces: header contains the "early-only" option, so
that the pickup operation fails if extension 123 answers the call.
6. As a consequence of executing the INVITE/Replaces, the caller
sends a CANCEL of its INVITE 1 to phone 123.
7. The caller sends a 200 response to the INVITE 5 from phone 456.
At this point, Caller is talking to phone 456.
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3. Detailed call flow
A more complete packet diagram of a SIP-B pickup sequence is as
follows. (100 responses are present only if the transport is UDP, and
are omitted on this diagram.) Messages in the same transaction are
labeled with the same number but successive letters, e.g., 7a, 7b,
7c.
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Caller Target Executing
123 456
| | |
| 1a INVITE | |
|----------------------->| |
| | |
| 1b 180 Ringing | |
|<-----------------------| |
| | |
| | | 2 Call Pickup
| | | on 123
| | |
| | 3a SUBSCRIBE |
| |<-------------------|
| | |
| | 3b 200 OK |
| |------------------->|
| | |
| | 4a NOTIFY |
| |------------------->|
| | |
| | 4b 200 OK |
| |<-------------------|
| | |
| 5a INVITE/Replaces | |
|<--------------------------------------------|
| | |
| 5b 200 OK | |
|-------------------------------------------->|
| | |
| 5c ACK | |
|<--------------------------------------------|
| | |
| 1c CANCEL | |
|----------------------->| |
| | |
| 1d 200 OK | |
|<-----------------------| |
| | |
| 1d 487 Request Term. | |
|<-----------------------| |
| | |
| 1e ACK | |
|----------------------->| |
| | |
| | |
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1. The caller (AOR Caller@example.com) sends an INVITE 1a with URI
123@example.com to phone 123.
2. The user of phone 456 activates the call pickup feature for
extension 123. This may be activated by a special button on the
phone, or through a dialing prefix which is intercepted by the
phone.
3. Phone 456 sends a SUBSCRIBE 3a with URI 123@example.com to
extension 123, requesting "Event: dialog" and "Expires: 0". This
SUBSCRIBE requests one NOTIFY containing the status of all dialogs
involving AOR 123@example.com.
This requires that the other phones in the installation can
produce dialog event notifications. We discuss below using a
state agent to support phones that cannot produce dialog event
notifications.
Since dialog event notifications can contain sensitive
information, the ability to SUBSCRIBE to them should be controlled
by authentication. This will be discussed below.
4. Phone 123 sends one NOTIFY 4a to phone 456 giving the status of
its current dialogs for AOR 123@example.com, which includes the
early dialog of INVITE 1a from Caller, and gives the "remote
identity" and/or "remote target" of the dialog (which are the
From: and Contact: of INVITE 1a), which is "Caller@example.com".
5. Phone 456 sends to Caller@example.com an INVITE 5a. It has a
Replaces: header specifying the dialog parameters sent in NOTIFY
4a. Its URI is the remote target address, and its has a Replaces:
header specifying the dialog parameters sent in the NOTIFY.
If the NOTIFY 4a does not include a "remote target" URI (which
should always identify the caller UA), the executing phone will
have to fall back to the "remote identity" URI (in hopes that it
routes to the caller UA). The "remote target" URI should be the
Contact URI from INVITE 1a. In order for INVITE 5a to succeed,
that Contact needs to be globally routable.
1c. The caller sends a CANCEL 1c of its INVITE 1a to phone 123.
1d. The caller sends a 200 response to the INVITE 5a from phone 456.
At this point, Caller is talking to phone 456.
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4. Call pickup with proxy
Now let us consider call pickup when the organization has a proxy for
routing calls.
Caller Proxy Target Executing
123 456
| | | |
| 1a INVITE | | |
|-------------------->| | |
| | 1b INVITE | |
| |--------------->| |
| | 1c 180 Ring. | |
| |<---------------| |
| 1d 180 Ringing | | |
|<--------------------| | |
| | | |
| | | | 2 Call
| | | | Pickup
| | | | on 123
| | | |
| | | 3a SUBSCRIBE |
| |<---------------------------------|
| | 3b SUBSCRIBE | |
| |--------------->| |
| | 3c OK | |
| |<---------------| |
| | | 3d 200 OK |
| |--------------------------------->|
| | | |
| | | 4a NOTIFY |
| | |---------------->|
| | | 4b 200 OK |
| | |<----------------|
| | | |
| 5a INVITE/Replaces | | |
|<-------------------------------------------------------|
| 5b 200 OK | | |
|------------------------------------------------------->|
| 5c ACK | | |
|<-------------------------------------------------------|
| | | |
| 1e CANCEL | | |
|-------------------->| | |
| | 1f CANCEL | |
| |--------------->| |
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| | 1g 200 OK | |
| |<---------------| |
| 1h 200 OK | | |
|<--------------------| | |
| | 1i 487 Req.T | |
| |<---------------| |
| 1j 487 Req. Term. | | |
|<--------------------| | |
| 1k ACK | | |
|-------------------->| | |
| | 1l ACK | |
| |--------------->| |
| | | |
| | | |
The major differences from the previous example are:
1. INVITE 1a and later messages in its transaction go through the
proxy, thus allowing the proxy to convert 123@example.com to a
series of contact points.
3. SUBSCRIBE 3a also goes through the proxy, so it is also sent to a
series of contact points. The proxy should fork the SUBSCRIBE in
the same way as it would fork an INVITE, except that it should
fork all the contact points in parallel, even if they would fork
serially for INVITEs, so phone 456 cab find its target call
quickly.
As SUBSCRIBE 3a is forked, its request URI is changed from the AOR
123@example.com to various contact URIs, usually the same user
name with host parts that are the IP addresses of the phone user
agents. This will match the request URIs of the INVITEs 1b, so
information about those dialogs will be reported in NOTIFYs.
4. Because SUBSCRIBE 3a can be forked, phone 456 should be prepared
to receive multiple NOTIFYs from various phones for AOR 123, and
be able to select the correct dialog if it discovers more than
one.
Phone 456 should beware that it may see multiple instances of a
single forked INVITE (which have the same Call-Id), and not
present them to the user as alternatives.
Phone 456 should beware that one AOR might be forwarded to a URI
with a different user name. E.g., "123@example.com" might fork
first to an user agent on 123's desk ("123@10.1.200.12") and then
to a back-up responder ("789@example.com"). However, a UA that
receives SUBSCRIBE 3a will see the same request URI as it saw when
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it received INVITE 1a, and thus will include any dialog it creates
in its subsequent NOTIFY 4a. But it may include dialogs that were
originally sent to other AORs as well. Unfortunately, there is no
clean way to distinguish the dialogs that were originally for 123,
as the best approximation to that information is the "local
identity" (i.e., "To:" URI), but that may have originally been an
address that forwards to 123. In the end, we should avoid to
forwarding two AORs to the same request-URI, so as to avoid this
ambiguity.
5. The INVITE/Replaces 5a may be forwarded through the proxy as well.
Doing so makes no essential difference to the operation.
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5. Call pickup with proxy and state agent
This case is when the phones themselves cannot generate dialog event
notices, so that work is delegated to a state agent which receives
information from the site's proxy.
Caller Proxy State Target Executing
Agent 123 456
| | | | |
| 1a INVITE | | | |
|--------------->| | | |
| | 1b INVITE | | |
| |-------------------------->| |
| | 1c 180 Ring. | | |
| |<--------------------------| |
| 1d 180 Ringing | | | |
|----------------| | | |
| | | | |
| | State Info. | | |
| |++++++++++++++>| | |
| | | | |
| | | | | 2 Call
| | | | | Pickup
| | | | | on 123
| | | | |
| | | | 3a SUBSCRIBE |
| |<-----------------------------------------|
| | 3b SUBSCRIBE | | |
| |-------------->| | |
| | 3c OK | | |
| |<--------------| | |
| | | | 3d 200 OK |
| |---------------|------------------------->|
| | | | |
| | | 4a NOTIFY | |
| | |------------------------->|
| | | 4b 200 OK | |
| | |<-------------------------|
| | | | |
| | | | |
[Remainder of operation is as before.]
The major differences in processing are:
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The proxy monitors the progress of dialogs and reports state
information via a possibly non-SIP channel to the state agent.
Since the state agent needs to report only early dialog
information, one possible channel is to do a non-blocking fork to
the state agent of every call to 123. (This trick was invented by
Scott Lawrence.) However, if the state agent will be used to
support applications that require knowledge of committed dialogs
(e.g., attendant consoles), this technique alone will not be
sufficient.
3. The proxy routes SUBSCRIBE 3a not to phone 123 but rather to the
state agent. The state agent uses the request URI or the From:
URI to determine the user on whose behalf it should generate
events.
Since a SIP call can be maintained indefinitely without any
signaling, it is possible for the state agent to miss the
termination of a call and believe that the call is continuing
forever. There needs to be a way to clear such state. Early
dialog state can be cleared due to the global time limit on INVITE
transactions, but committed dialogs have no intrinsic time limit.
One way to reset the state agent would be to define a dialing
prefix, and any call to an extension prepended with the dialing
prefix would be routed to the state agent, which would interpret
the INVITE as a request to reset the state of the specified
extension.
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6. Call pickup with a pickup agent
A further case allows calls to be picked up by phones that do not
implement SIP-B pickup. Essentially, the phone makes a call that is
routed to a "pickup agent" which executes the SIP-B pickup process on
the phone's behalf.
Caller Target pickup Executing
123 Agent 456
| | | |
| 1a INVITE | | |
|------------------>| | |
| 1b 180 Ringing | | |
|<------------------| | |
| | | |
| | | | 2 Dials
| | | | *78123
| | | |
| | | 3a INVITE |
| | |<--------------|
| | | |
| | 4a SUBSCRIBE | |
| |<----------------| |
| | 4b 200 OK | |
| |---------------->| |
| | | |
| | 5a NOTIFY | |
| |---------------->| |
| | 5b OK | |
| |<----------------| |
| | | |
| 3b INVITE/Replaces | |
|<------------------------------------| |
| 3c 200 OK | | |
|------------------------------------>| |
| | | 3d 200 OK |
| | |-------------->|
| 3e ACK | | |
|<----------------------------------------------------|
| | | |
| | | |
[Remainder of operation is as before.]
The major differences in processing are:
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2. The executing user dials a special prefix and then the extension
from which the call is to be picked up.
3. Because the dialed number starts with the pickup prefix "*78", the
call is routed (either by a phone dialplan or an outgoing call
proxy) to the pickup agent.
4 and 5. The pickup agent executes the same sequence of actions as a
SIP-B phone would.
3b. After determining the Caller's URI, the pickup agent appends a
Replaces: header to the INVITE and forwards it as if it was
proxying the call.
If the target phone is represented by a state agent, then the
pickup agent and the state agent could communicate by non-SIP
means rather than using SUBSCRIBE and NOTIFY.
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7. With improved pickup agent
The previous packet flow is suboptimal in that the pickup agent acts
as a non-standard proxy, rewriting an INVITE into an INVITE with
Replaces: header. We can adopt a technique from Dan Petrie to be
strictly conformant: The pickup agent returns a 302 (Moved
Temporarily) response to the executing phone, bearing a Contact:
header with a URI <caller-URI?Replaces=dialog-parameters>.
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Caller Target Pickup Executing
123 Agent 456
| | | |
| 1a INVITE | | |
|-------------------->| | |
| 1b 180 Ringing | | |
|<--------------------| | |
| | | |
| | | | 2 Dials
| | | | *78123
| | | |
| | | 3a INVITE |
| | |<--------------|
| | | |
| | 4a SUBSCRIBE | |
| |<---------------| |
| | 4b 200 OK | |
| |--------------->| |
| | | |
| | 5a NOTIFY | |
| |--------------->| |
| | 5b OK | |
| |<---------------| |
| | | |
| | | 3b 302 |
| | | ...?Replaces |
| | |-------------->|
| | | |
| 4a INVITE/Replaces | | |
|<-----------------------------------------------------|
| 4b 200 OK | | |
|----------------------------------------------------->|
| 4c ACK | | |
|<-----------------------------------------------------|
| | | |
| | | |
[Remainder of operation is as before.]
The major differences in processing are:
3. Instead of forwarding the INVITE to the Caller, the pickup agent
returns a 302 response, whose Contact is a URI for the Caller with
a Replaces header parameter specifying dialog 1a. Phone 456 then
sends INVITE/Replaces 4a to Caller.
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8. With proxy and improved pickup agent
Let's combine some of the above flows to show a realistic case, where
all signaling goes through the proxy, and we use the "improved"
pickup agent to support a phone that does not do call pickup by
itself.
Caller Proxy Target Pickup Executing
123 Agent 456
| | | | |
| 1a INVITE | | | |
|--------------->| | | |
| | 1b INVITE | | |
| |----------->| | |
| | 1c 180 Rin.| | |
| |<-----------| | |
| 1d 180 Ringing | | | |
|<---------------| | | |
| | | | |
| | | | | 2
| | | | | Dials
| | | | | *78123
| | | | |
| | | | 3a INVITE |
| |<----------------------------------------|
| | 3b INVITE | | |
| |-------------------------->| |
| | | | |
| | | 4a SUBSCRIBE | |
| |<--------------------------| |
| | 4b SUBSC. | | |
| |----------->| | |
| | 4c 200 OK | | |
| |<-----------| | |
| | | 4d 200 OK | |
| |-------------------------->| |
| | | | |
| | 5a NOTIFY | | |
| |<-----------| | |
| | 5b NOTIFY | | |
| |-------------------------->| |
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| | | 5c 200 OK | |
| |<--------------------------| |
| | 5d 200 OK | | |
| |----------->| | |
| | | | |
| | 3c 302 Contact: <Caller?Replaces=xxx> |
| |<--------------------------| |
| 3e INVITE/Repl.| | | |
|<---------------| | | |
| 3f 200 OK | | | |
|--------------->| | | |
| | 3g 200 OK | | |
| |---------------------------------------->|
| | 3h ACK | | |
| |<----------------------------------------|
| 3i ACK | | | |
|<---------------| | | |
| | | | |
| 1e CANCEL | | | |
|--------------->| | | |
| 1f 200 OK | | | |
|<---------------| | | |
| | 1g CANCEL | | |
| |-------------------------->| |
| | 1h 200 OK | | |
| |<--------------------------| |
| | 1i 487 Req. Term. | |
| |<--------------------------| |
| 1j 487 Req. Term | | |
|<---------------| | | |
| 1k ACK | | | |
|--------------->| | | |
| | | 1l ACK | |
| |-------------------------->| |
| | | | |
| | | | |
The overall call flow is:
1. Caller sends invite to the target (123).
2. Executing phone (456) dials *78123.
3. Due to the dialing prefix, the INVITE is routed to the pickup
agent.
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4. The pickup agent recognizes the dialing prefix *78 and puts the
transaction aside for a moment. It initiates a SUBSCRIBE with
"Expires:0" (one-time) to 123.
5. The target sends a NOTIFY back to the pickup agent.
3d. The pickup agent selects dialog 1 to be picked up, and returns a
302 for INVITE 3b to the proxy. The 302 contains "Contact:
<caller-URI?Replaces=dialog-1-parameters>".
3e. The proxy starts another fork of INVITE 3a, sending an INVITE 3e
with Replaces header to the Caller. INVITE 3e is accepted,
replacing the dialog initiated by INVITE 1a. Caller then sends
CANCEL 1e for INVITE 1a, leading to a 200 OK response for the
CANCEL and a 487 response for INVITE 1a, as well as an ACK for the
487 response.
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9. Group call pickup
There are three kinds of group call pickup:
9.1. Picking up any call to any member of a group of individuals
This is a pickup directed to "any call to any of the people in my
group", that is, being able to pick up a call without having to first
determine the specific extension to which it was directed.
If the proxy is configured to provide a URI that forwards to all
members of the group, a SUBSCRIBE to that URI will be forwarded to
all of the relevant user agents. However, no incoming call contains
the group URI as part of its dialog identification, so it becomes
questionable how the executing phone determines that it can pickup
the call.
The common convention is that a pickup that has several possible
targets should pick up the call that has been waiting longest. The
dialog event package contains a "duration" parameter.
9.2. Picking up a call routed to several individuals
In terms of the old key phone systems, this situation is like an
extension number that rings on several people's multi-line phones.
This can be handled without additional work, because the proxy will
already be routing this URI to all of the individuals' user agents,
and so it will route a SUBSCRIBE to that URI to all of the user
agents as well, ensuring that the executing phone will see
information about the incoming call.
9.3. Picking up a call that is not routed to a specific user agent
This is the situation of a call when the caller dials a number that
is not immediately routed to any particular phone, but rather "rings
in limbo" until someone picks it up. In SIP, this service is
implemented via an intangible user agent for the AOR that is the
endpoint of an early dialog, but never answers it.
Call pickup from this sort of "calling group" is very similar to call
pickup from a phone or individual line, as long as the intangible UA
implements the needed SIP features to participate in a call pickup.
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10. Call park
Retrieving a call from call parking is very much like call pickup of
a ringing call.
To park a call, the phone transfers it to an extension that is
maintained by a "park server", a multi-line software UA that accepts
and maintains dialogs to a set of extensions that are defined as
"parking orbits".
To retrieve a parked call, a phone executes a call pickup operation,
except that the parameters of the Replaces header do not include the
"early-only" operation, so that the confirmed dialog with the park
server can be retrieved.
When call pickup and call retrieve are implemented by a pickup agent,
the pickup agent is the only component that needs to distinguish the
parking orbits and generate the Replaces header specification in its
302 response differently.
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11. Security considerations
All of these mechanisms (with the exception of using an integrated
state agent and pickup agent) depend on SUBSCRIBE/NOTIFY for the
dialog event package. But SUBSCRIBE reveals information which could
be sensitive -- we have just shown that anyone who can successfully
SUBSCRIBE for an extension can take calls from that extension!
Thus, we need to ensure that the acceptance of SUBSCRIBEs is
controlled by suitable security mechanisms.
12. Informative References
[1] Salzer, J., Reed, D., and D. Clark, "End-to-End Arguments in
System Design", November 1984.
ACM TOCS, Vol 2, Number 4, November 1984, pp 277-288.
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Author's Address
Dale R. Worley
Pingtel Corp.
400 West Cummings Park, Suite 2200
Woburn, MA 01801
US
Phone: +1 781 938 5306 x173
Email: dworley@pingtel.com
URI: http://www.pingtel.com
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Internet-Draft Call Pickup Examples in SIP February 2006
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