Re: Mirja Kühlewind's Discuss on draft-ietf -mmusic-sctp-sdp-23: (with DISCUSS)

Hi All,

I think a little bit of background will help here.

UDP/DTLS/SCTP and TCP/DTLS/SCTP are designed to work with ICE (RFC 5245).

In ICE environments, during the nomination process, end points go through
multiple candidate pairs, until the most preferred pair is found. During
this selection process, data can be sent as soon as the first working pair
is found, but the process still continues and candidate pairs can change
while data is sent. Furthermore, if end points roam, for instance when
mobile end point switches from mobile internet to wifi, end points will
initiate an ICE restart, which will trigger a new nomination process
between the new set of candidates and likely result in new nominated
candidiate pair. When these candidates change, the same DTLS association
continues to run, regardless whether it is running over udp or tcp
candidate pair. Because of this, ICE tcp requires using RFC 4571 framing
when sending data (https://tools.ietf.org/html/rfc6544#section-10.1).
Otherwise, if TLS were used, a new TLS or DTLS session would be required
every time candidate pair switches between tcp and udp candidates. In order
to simplify transition between different underlying transports, DTLS is
used for both udp and tcp candidates and TCP/DTLS/SCTP transport tag is
defined to differentiate it from other protocols.

As far as TCP/DTLS/SCTP transport tag is concerned, please note that ICE
end points are supposed to send a re-INVITE after nomination process is
completed with the selected candidate address in the m= line. So, if tcp
candidate is selected, re-INVITE must be sent with TCP/DTLS/SCTP transport
tag in the m= line. Also, any offers/answers after the ICE nomination is
complete, are supposed to send the currently selected candidate in the m=
line, which will also be TCP/DTLS/SCTP in case tcp candidate is selected.

I hope this addresses your concern and explains why TCP/DTLS/SCTP is
defined instead of TLS/SCTP.

Regards,
_____________
Roman Shpount

On Thu, Feb 16, 2017 at 10:24 AM, Christer Holmberg <
[email protected]> wrote:

> Hi,
>
>
>
> …
>
>
>
> >The only question is what should appear in the m= proto line.
>
>
>
> Even if nobody puts it in the m= line, I think it’s useful to keep it in
> the document, as it gives an overview how it’s realized etc.
>
>
>
> It doesn’t cause any harm to keep it, and it’s “future proof” IF someone
> wants to use it without ICE at some point.
>
>
>
> Regards,
>
>
>
> Christer
>
>
>
>
> > Am 16.02.2017 um 16:02 schrieb Eric Rescorla <[email protected]>:
> >
> > As Christer says. This design is optimized for making the media stack
> simpler, which
> > using TLS here would not do.
> >
> > -Ekr
> >
> >
> >
> >
> > On Thu, Feb 16, 2017 at 7:00 AM, Christer Holmberg <
> [email protected]> wrote:
> > Hi,
> >
> > ----------------------------------------------------------------------
> > DISCUSS:
> > ----------------------------------------------------------------------
> >
> > >>> Why is this using TCP/DTLS/SCTP instead of TCP/TLS/SCTP?
> > >>>
> > >> Because the way it is realized is by transporting SCTP on top of DTLS
> (as defined in draft-ietf-tsvwg-sctp-dtls-encaps) and
> > >> transporting DTLS on top of TCP (defined in RFC 4571).
> > >
> > > I got this but DTLS is a mapping to use TLS with UDP because UDP is an
> unreliable datagram transport. If you use TCP, you
> > > should use TLS. And rfc4571 is not a mapping of DTLS to TCP.
> >
> > The framing mechanism of RFC 4571 is used, with DTLS packets sent
> instead of RTP packets.
> >
> > Regards,
> >
> > Christer
> >
> >
>
>
>

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