[SIPForum-techwg] SIP Forum Recommendation for Call Hold Feature.
andrew.hutton at siemens.com
Tue Sep 13 03:57:03 EDT 2005
One of the things our working group proposal indicates we might work on is
the production of some recommendations for SIP Protocol feature
requirements. I am not sure what form such a proposal will take but I
thought I would test the water by suggesting that one of the features we
should be writing a recommendation for is Call Hold.
Putting a call on hold is one of the basic features of any telephone and
every SIP phones I have seen has implemented some kind of call hold function
however there are some aspects of this feature which in a SIP environment
are open to interpretation and therefore interoperability problems. It is
also my understanding that this is not a feature that the IETF will tackle.
A SIP Forum recommendation would be useful to set the baseline requirements
for a SIP Phone to support Call Hold.
Issues which a SIP Forum recommendation on call hold could help to clarify
1. How does a SIP Phone indicate a Call Hold invocation (i.e. use of
a=sendOnly, a=inactive etc.).
2. How is Music on Hold invoked either from a phone or via a media server.
3. How does hold apply to multiple media streams (e.g. audio and video).
4. RTCP Issues.
5. Use of the dialog-event-package "sip.rendering" parameter to indicate
Another question I have is whether this working group will make
recommendation purely for SIP Phones or whether we will also include
recommendations for other entities like Media Servers and Application
Servers as in most environments these servers do exist and impact the SIP
I apologise for starting with one of the less interesting features but I
think this could be a useful starting point and is a feature we need to
Andrew Hutton - System Design Consultant
Tel: +44 (0)115 943 2618
Mobile: +44 (0) 7711877253
e-mail: mailto:andrew.hutton at siemens.com <mailto:andrew.hutton at siemens.com>
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