Re: [MMUSIC] Connection Data Capability (ccap) and IP-addresses (draft-ietf-mmusic-sdp-miscellaneous-caps-04)

Jonathan Lennox <> Fri, 22 March 2013 20:00 UTC

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From: Jonathan Lennox <>
To: Flemming Andreasen <>
Date: Fri, 22 Mar 2013 16:00:22 -0400
Thread-Topic: [MMUSIC] Connection Data Capability (ccap) and IP-addresses (draft-ietf-mmusic-sdp-miscellaneous-caps-04)
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Subject: Re: [MMUSIC] Connection Data Capability (ccap) and IP-addresses (draft-ietf-mmusic-sdp-miscellaneous-caps-04)
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Currently, the deployed SIP world has three mechanisms to allow some flavor of negotiation among multiple IP addresses: ICE, altc (despite the general disapproval of the working group), and ANAT (despite its deprecation).

I think that adding ccap as a fourth member of this set would be a terrible idea; and as far as I can tell, no one wants to do that.  So we need to make it clear that that it MUST NOT be used for that purpose.

In the formulation below, I think I'd say that a given media description MUST NOT indicate more than one address with an IN network type, across all its configurations (actual and potential).

Obviously, different media descriptions (m= line blocks) can have different addresses.

In practice, given the port number issue that started this thread, I suspect this means that the SDP offer will need to put the IN address in the actual configuration (in the c= line), and the PSTN address(es) will be in the potential configurations.

On Mar 22, 2013, at 2:37 PM, Flemming Andreasen wrote:

> Still waiting for more comments on this, especially from the people that 
> were very vocal in their complaints previously: Now is the time to speak up.
> Regardless, a few comments on the below:
> 1) It allows the use of "ccap" to be used to indicate one or more "IP4" 
> addresses in a given SDP.
> 2) It allows the use of "ccap" to be used to indicate one or more "IP6" 
> addresses in a given SDP.
> Nit-picking a bit on the actual text, which I think is important:
> The "ccap" attribute is not what is being to select between different 
> IP-addresses; the use of a "ccap" attribute in a potential configuration 
> ("pcfg") is what is being used for this. Is the restriction that we want 
> here:
> a) A potential configuration MUST NOT reference more than one "ccap" 
> attribute with a network type of "IN" ?
> b) All potential configurations for a particular media description MUST 
> NOT reference more than one "ccap" attribute with a network type of "IN" ?
> c) Something else ?
> Thanks
> -- Flemming
> On 3/22/13 1:35 AM, Andrew Allen wrote:
>> I am OK with either of these proposals
>> -----Original Message-----
>> From: [] On Behalf Of
>> Sent: Wednesday, March 20, 2013 5:57 AM
>> To:;
>> Subject: Re: [MMUSIC] Connection Data Capability (ccap) and IP-addresses (draft-ietf-mmusic-sdp-miscellaneous-caps-04)
>> I went through the discussion, and my reading is that there is agreement on not allowing ccap to be used for alternative IP address negotiation.
>> That could be made clear in the text e.g. by modifying the second sentence Flemming quoted to read:
>> <quote>
>>     The 'ccap' attribute MUST NOT be used to select
>>     between different IP connection addresses (e.g. between
>>     "IP4" and "IP6" address families or different IP addresses
>>      within the same IP address family).
>> </quote>
>> The ccap attribute should be able to carry either an IP or PSTN address; that way either a PSTN or an IP bearer could be offered as the highest priority configuration (in the "m=" line).  However, if we want to clarify the intended use of ccap, we could modify the first sentence to read:
>> <quote>
>>    The 'ccap' capability attribute is intended for offering
>>    alternative connection addresses where the <nettype>
>>    is "IN" or "PSTN", i.e. selecting between an IP based
>>    bearer or a circuit-switched bearer.
>> </quote>
>> Simo
>> -----Original Message-----
>> From: [] On Behalf Of ext Flemming Andreasen
>> Sent: 19. maaliskuuta 2013 8:24
>> To: mmusic
>> Subject: [MMUSIC] Connection Data Capability (ccap) and IP-addresses (draft-ietf-mmusic-sdp-miscellaneous-caps-04)
>> Greetings
>> As you may have seen, there has recently been some list discussion on the "connection data capability" defined in
>> draft-ietf-mmusic-sdp-miscellaneous-caps-04 (see e.g. thread in
>> To recap, the connection data capability ("ccap") provides capability negotiation capabilities for what amounts to the "c=" line in regular SDP, and as such enables negotiation of network type (such as "IN") and IP-address information (v4 and v6 addresses). The Standards Track mechanism for negotiating and determining alternative IP-address information today is ICE, and hence the draft currently includes the following wording:
>> <quote>
>> The 'ccap' capability attribute is intended to
>>     be used only when there is no other mechanism available for
>>     negotiating alternative connection address information, such as when
>>     the <nettype> is different among the alternative addresses (e.g.
>>     "IN" and "PSTN").  The 'ccap' attribute MUST NOT be used in
>>     situations where an existing mechanism (such as Interactive
>>     Connectivity Establishment (ICE) [RFC5245]) can be used to select
>>     between different connection addresses (e.g.  "IP4" and "IP6" or
>>     different IP addresses within the same IP address family).
>> </quoted>
>> The above text has led to some confusion as to exactly when and what "ccap" can be used for. More specifically, is it/should it ever be allowed to use "ccap" to convey an IP4 or IP6 address, and if so, under what circumstances ?
>> If you have an opinion, please let us know.
>> A couple of points to keep in mind:
>> - The current document has been WGLC'ed without comment ~6 months ago.
>> - 3GPP has a dependency on the document (however I'm not sure if that dependency includes the above "IN" feature)
>> - The connection data capability is defined in a general manner to be generally useful in line with the overall capability negotiation framework (as opposed to targeted at one specific use case with one specific value)
>> - There are scenarios where ICE cannot be used, even if implemented (e.g. ice-mismatch).
>> - RFC 6849 (media loopback) provides for NAT traversal in the absence of ICE support
>> Thanks
>> -- Flemming
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Jonathan Lennox