RCS a standard to supercede SMS and Threema?

Harald Welte laforge at gnumonks.org
Fri Mar 26 19:38:59 UTC 2021

Hi Bernhard,

On Thu, Mar 18, 2021 at 12:16:59PM +0100, Bernhard E. Reiter wrote:
> did you know that 
>   https://en.wikipedia.org/wiki/Rich_Communication_Services
> is a standard that aims at replacing SMS,

yes - but then I guess I'm as deep as one can get in terms of telecom
protocols and specs ;)

> After discovering this yesterday, I wondered, why didn't I hear about this 
> before? 

For sure within the telecom operator domain it is not a new topic, as you can
see from the number of deployments during the past couple of years.  It's the
desperate attempt by operators to not loose all of the SMS revenue to
internet messaging services.

> Does somebody know more?

Unfortunately I've never studied it in detail, at Osmocom we mostly work on
the lower-layer protocol stacks.

>   * Is this really an open standard (like we define it [1])

It is specified by 3GPP and OMA, both organizations have a long history of fully
publishing the standards to the general public free of charge.  However, their
specs are normally heavily patent-encumbered and you will need to obtain patent
licenses from the individual patent holders.  Or simply not care and wait until
you become relevant enough to be sued.

>   * Why isn't Apple participating yet?

As if apple would ever want to voluntarily participate in an open, interoperable,
standards-based system, if they can just as well continue to push their own walled

In general there is the obvious struggle between:

* the large internet corporations wanting to lcck their users into whatever is their
  own walled garden

* the mobile operators who use to be able to provide telephony and text message services
  based on interoperable international standards for many decades.  RCS is their approach
  to translate this into the 21st century.

>   * Can non-Google phones run it (Like /e/, LineageOS-MicroG or SailfishOS,
>     iOS)

Technically, RCS is based on TCP/IP and uses the services of IMS.  IMS is the IP
Multimedia System, which is what implements voice calls (and optionally SMS)
for VoLTE and Vo5G.

Normally, the modem establishes a separate second IP tunnel/connection to the
network, which is not the one you use for public IP access from your apps or for
tethering / mobile hot spot.

That second IP tunnel has its own phone-side IP address, and typically even uses
an additional layer of encryption via IPsec.  Over that there is SIP for VoLTE,
and also RCS.

The problem is now that while cellphones finally use IETF-derived VoIP protocols
for telephony, all of this is not implemented on the application processor but
typically inside the modem.  This means unless you know how to access this separate
IP tunnel used for IMS from the application processor, you would not be able to
write a FOSS or 3rd party RCS client.

> ps.: Does someone remember "co -l" "ci" with real rcs? >;)

I actually still use it occasionally!

- Harald Welte <laforge at gnumonks.org>           http://laforge.gnumonks.org/
"Privacy in residential applications is a desirable marketing option."
                                                  (ETSI EN 300 175-7 Ch. A6)

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