[Freeswitch-users] OT: Apple goes to great lengths to defeat NAT and firewalls with new Facetime

[Steven Ayre]

>
> Wondering why the FS developers default to not including rtpmap lines
> for statically defined RTP payload types?


Because SIP over UDP doesn't play well with fragmentation. SDP can make
packets large than the PMTU, leading to fragmented packets, which leads to
devices ignoring the packet. Removing the unnecessary rtpmap lines means
smaller SDP so smaller packet so less likelihood of that being an issue.

It shouldn't be such an issue over TCP.

Devices *should* support it since the standard explicitly say the rtpmap
isn't required for static types, but there are some manufacturers who
ignored that part so there's the verbose_sdp=true compatibility option for
them.

Wonder why SIP signalling over TCP or TLS is much more reliable

through various NAT and firewall devices?


That's not necessarily a given.

In general though because the TCP connection explicitly signals the
connection closing the mapping will stay in the firewall. With UDP it is
removed after a long period of inactivity. That can cause problems with
signalling during a phone call unless the endpoints send keepalive packets
often enough.

TLS will prevent the router helping with SIP ALG - you must have endpoints
capable of doing NAT traversal themselves (STUN). Though that's a good idea
in all cases anyway.

I wonder if FS could multiplex SIP and RTP over the same port someday?

Maybe support deflate encoding?


That wouldn't automatically work. It would need support by both ends and
protocol changes to support it.

What you're after is probably something like
http://tools.ietf.org/html/rfc5761 - patches welcome ;)





On 20 September 2013 14:07, Kristian Kielhofner <kris at kriskinc.com> wrote:

> Somewhat off-topic but because it comes up here regularly.
>
> Wondering why the FS developers default to not including rtpmap lines
> for statically defined RTP payload types?
>
> Wonder why SIP signalling over TCP or TLS is much more reliable
> through various NAT and firewall devices?
>
> Apple has put a significant amount of effort into redesigning Facetime
> to better handle NAT and firewall devices.  More details here:
>
> http://blog.krisk.org/2013/09/apples-new-facetime-sip-perspective.html
>
> I wonder if FS could multiplex SIP and RTP over the same port someday?
>  Maybe support deflate encoding?
>
> --
> Kristian Kielhofner
>
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