[openssl-users] How to handle DTLS Certificate Reassembly Error

Chad Phillips chad at apartmentlines.com
Sat Sep 17 15:11:02 UTC 2016

Matt, thanks for the reply, very helpful so far! Answers to your questions

You don't say what version of OpenSSL.

The support library I’m using is Licode:

The version of openssl I have compiled into it is 1.0.2h.

> The packet trace you sent is quite confusing, as there appears to be
> two separate handshakes going on at the same time that are interleaved.

Yes, apologies for that, I’ll do a better job of filtering the next time.
:) Those were two separate handshakes pulled from a packet capture of a
group video chat, filtered through Wireshark using the ‘dtls’ filter.

> It seems quite clear that this is a retransmit of the earlier message
> from client to server. Retransmits are a normal part of DTLS and are
> there to handle packet loss. If a retransmitted packet is received by
> one of the peers, and it has seen that packet before, then it is simply
> ignored. Wireshark isn't ignoring it, and is reporting it as an "error"
> simply because it has seen it before.

Thanks for that clarification.

> Was this packet capture done on the client side or the server side or
> somewhere in the middle? There appears to be some messages missing.
> In particular I don’t see any CCS or Finished messages being exchanged.
> Is the network this is over potentially noisy that might explain packet
> loss?

>From the perspective of the DTLS handshake, my server hosting the Licode
library is the client, and latest stable Chrome browser is the server, if I
understand the terminology correctly. The packet capture was taken from the
client (Licode) side.

Would the CCS or Finished messages have gotten filtered out by the ’dtls’
filter I applied to the packet capture? I do have the full trace and can
re-filter to just one complete connection over a specific UDP port as you
suggested, let me know if that would be helpful

I see these failures only in situations where browser users with slow
and/or lossy internet are joining, and usually when the group size gets to
be six or more participants. The particular testing scenario that generated
the packets you saw was a user with 225kbps upload, 5120kbps down, 70ms
delay, 0% packet loss.

I’ll grant you those network conditions aren’t the best for group video
chat, but if Google Hangouts can pull it off, I’d like to as well.

> On receiving that the client should respond with a retransmit of
> the Certificate/ClientKeyExchange/CertificateVerify/CCS/Finished
> flight of messages. But it does not appear to do so…the retransmit does
> not happen until after the encrypted alert.

This sounds like it might be a bug in the Licode library, not resending the
retransmit properly?

> Are both ends of the communication using OpenSSL and if so what versions?

>From my research, I believe Chrome uses borringssl?
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