[erlang-questions] DTLS: doubts about fragment reassembler

Andreas Schultz aschultz@REDACTED
Thu Mar 9 19:54:36 CET 2017

Hi Ingela,

I tried to understand the fragment reassembler in dtls_handshake.erl
and it seems that it is assuming that retransmissions will always use
the same record size.

RFC 6347, Sect. 4.3.2 states:

> When a DTLS implementation receives a handshake message fragment, it
> MUST buffer it until it has the entire handshake message.  DTLS
> implementations MUST be able to handle overlapping fragment ranges.
> This allows senders to retransmit handshake messages with smaller
> fragment sizes if the PMTU estimate changes.

Last time I looked at OpenSSL's DTLS sending side, it did exactly that.
It refragmented the handshake on retransmit to the minimum IP MTU.

>From my understanding of the fragment merging logic, it will not merge
overlapping fragments (it might even become confused).

Am I missing something there?


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