Re: [TLS] DTLS/SCTP and fragmentation

Rick van Rein <rick@openfortress.nl> Mon, 05 April 2021 12:13 UTC

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Date: Mon, 05 Apr 2021 14:12:58 +0200
From: Rick van Rein <rick@openfortress.nl>
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Subject: Re: [TLS] DTLS/SCTP and fragmentation
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Hi,

Larger frames than the MTU are not just a problem to Diameter; they also
complicate the normal handshake in DTLS which is a bit of a misfit with
DTLS delivery semantics.

Since the version is bit-swapped in DTLS, each record can easily be
distinguished as being either DTLS or TLS.  Then, why not allow the
mixing of those records in a stream, and map them differently to the
transport protocol?

I suppose the records could be marked as being the first and/or last in
a large user message, and this could be meaningfully translated to
properties and behaviour of the transport.  Below the DTLS MTU,
information is sent as DTLS, and above it, it is sent as a sequence of
TLS frames -- or are rejected, if the transport cannot handle that.
Plain TLS could be a special case where the DTLS MTU is set to 0.

Datagrams may have a number of meanings, too, that translate to
different transport meanings.  Diameter differs from RTP in that it
wants reliable delivery (which is why it does not carry over UDP) but it
is like RTP in that it does not want ordered delivery.  Plain TLS
applications would present the usecase of reliable ordered delivery.


Hopefully helpful,
 -Rick