Subject: [MMUSIC] FQDN resolution with SDP

From: Peter Saint-Andre <[email-masked]>
Date: Thu, 15 Apr 2010 11:40:42 -0600

I lodged a DISCUSS on draft-ietf-mediactrl-sip-control-framework-11
regarding resolution of FQDNs found c= lines, but Robert Sparks pointed
out that this issue is broader than that I-D and suggested that I raise
it with the MMUSIC WG.

In particular:

1. I think that the IP4 and IP6 <addrtype> values are underspecified in
RFC 4566, because it's not clear there that an application resolves a
<connection-address> of FQDN via an A or AAAA lookup. There might even
be an opportunity for use of SRV records here, but that's not for me to
say. :)

2. Secondarily, RFC 4566 is a bit confusing about the fact that a domain
name is allowed as a value of <connection-address> -- see especially the
bullet points after "When the <addrtype> is IP4 and IP6" in Section 5.7
(which to a naive reader might imply that only an IP address is allowed
as a value of <connection-address>).

I'm happy to chat in more detail if that would be helpful.

The redacted text of my DISCUSS follows.

###

In the Overview and in Section 4.1, it is not clear how the client
determines the IP address for connecting to the server. The information
provided by the server is as follows in the Overview:

   c=IN IP4 mserver.example.com
   m=application 7563 TCP cfw

The Overview text states:

   The connection address (taken from 'c=') and port (taken from 'm=')
   are used to identify the remote port in the new connection.

And (in Section 4.1):

   The third sub-field for Connection Data is <connection-address>.
   This supplies a representation of the SDP originators address, for
   example dns/IP representation.  The address is the address used for
   connections.

But the client needs to connect to an IP address and cannot directly
make a TCP connection to a domain name or hostname. How does it resolve
mserver.example.com to an IP address? Via A or AAAA lookup? If the
document does not specify the client behavior then it will be impossible
to build interoperable implementations.

Also, allowing a domain name or hostname in "c=" appears to be in
conflict with Section 5.7 of RFC 4566, which allows only an IP address
as the value of the <connection-address> sub-field.

###

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From: "Kevin P. Fleming" <[email-masked]>
Date: Thu, 15 Apr 2010 12:57:31 -0500

>    example dns/IP representation.  The address is the address used for

In addition to your comments, the statement that the
<connection-address> is a representation of the "SDP originators (sic)
address" is not accurate either. The address in the c= line has no
relationship whatsoever to the address of the entity that is providing
the SDP; it is only the address that should be used for delivery of
media associated with the relevant media streams.

> as the value of the <connection-address> sub-field.

I would tend to think that only IP addresses are valid as the
<connection-address> sub-field, not domain names. The originator of the
SDP is providing a very specific target for the media stream, not
"anything that this name resolves to".

-- 
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
skype: kpfleming | jabber: [email-masked]
Check us out at www.digium.com & www.asterisk.org

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From: "Dan Wing" <[email-masked]>
Date: Tue, 20 Apr 2010 09:57:27 -0700

Here is some fuel for this fire:

FQDNs aren't used in the wild, only in IETF specs.

-d

> -----Original Message-----
> From: [email-masked] 
> [mailto:[email-masked]] On Behalf Of Peter Saint-Andre
> Sent: Thursday, April 15, 2010 10:41 AM
> To: [email-masked]
> Subject: [MMUSIC] FQDN resolution with SDP
>

---

From: Hadriel Kaplan <[email-masked]>
Date: Tue, 20 Apr 2010 15:43:07 -0400

Obviously the client must not be allowed use just DNS to resolve that c= line into A and AAAA records anyway, it has to use ICE.  ;)

-hadriel

> From: [email-masked] [mailto:[email-masked]] On Behalf
> Of Peter Saint-Andre
> Sent: Thursday, April 15, 2010 1:41 PM
> To: [email-masked]