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Subject: Re: gsmtap design/extensions?
From:   Guy Harris <guy@alum.mit.edu>
In-Reply-To: <20190413071227.GC24451@nataraja>
Date:   Sat, 13 Apr 2019 00:49:59 -0700
Cc:     Johannes Berg <johannes@sipsolutions.net>,
        Vadim Yanitskiy <axilirator@gmail.com>,
        OpenBSC Mailing List <openbsc@lists.osmocom.org>,
        Sean Tranchetti <stranche@codeaurora.org>, radiotap@netbsd.org,
        Dan Williams <dcbw@redhat.com>, netdev@vger.kernel.org,
        linux-wireless@vger.kernel.org,
        Aleksander Morgado <aleksander@aleksander.es>,
        Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>,
        =?utf-8?Q?Bj=C3=B8rn_Mork?= <bjorn@mork.no>
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To:     Harald Welte <laforge@gnumonks.org>
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On Apr 13, 2019, at 12:12 AM, Harald Welte <laforge@gnumonks.org> wrote:

> On Fri, Apr 12, 2019 at 07:15:56PM +0200, Johannes Berg wrote:
> 
>> Agree. Sorry about that. No disrespect was intended, but I'm still not
>> sure I understand the need for UDP encapsulation *as part of the
>> protocol*. I guess saying "GSMTAP can optionally be encapsulated in UDP
>> with the well-known port xyz" would be something else, and it'd make
>> more sense to me than saying it has to be.
> 
> Sure, like with most protocols you can wrap them in anything you want.
> 
> Let me put it like this:
> You don't have to run RTP inside UDP, you could equally put the RTP
> frames in to SCTP or DCTP.   It's just not what the original users of
> the protocol/spec had envisioned, but it can for sure be done, and has
> no side-effect other than not being interoperable with existing
> implementations.

Or you can just have LINKTYPE_RTP/DLT_RTP and supply them inside nothing.

However, unlike RTP, there is no reason *not* to do that for GSMTAP - it's not as if the IP or UDP headers in a packet from a host supplying GSMTAP-encapsulated packets provide any information necessary or even useful for dissecting the encapsulated packets.

Whether it's useful, or possible, to have any interfaces on a *host* with cellular modem connectivity supply the cellular-network traffic as packets with GSMTAP headers - which appears to be what Johannes is thinking of - is another matter (but even if the answer is no, there is, as per my other message, a use for a LINKTYPE_GSMTAP/DLT_GSMTAP header type).  That might not be possible, as cellular modems, as you note, tend to hide a lot of lower-layer details from the host.