[Linphone-developers] patch to fix ZRTP sequence number handling

[Eli Burke]

Attached is a patch to oRTP that fixes incorrect handling of ZRTP sequence numbers. We encountered a consistent error when connecting between Linphone and Groundwire using ZRTP (note: compatibility with Groundwire requires my previously submitted zrtp-hash patch). 

Bug details: Linphone sends ZrtpHello and receives Groundwire's ZrtpHelloAck, followed by ZrtpHello with a lower sequence number. This packet gets dropped with the error "Discarding received out of order zrtp packet:..." and Linphone never completes the ZRTP handshake. 

Though you may conclude that Groundwire is misbehaving, it is in fact in compliance with the RFC, which simply says:

The Sequence Number is a count that is incremented for each ZRTP
   packet sent.  The count is initialized to a random value.  This is
   useful in estimating ZRTP packet loss and also detecting when ZRTP
   packets arrive out of sequence.

http://tools.ietf.org/html/rfc6189#section-5

I have confirmed with Werner Dittmann (author of ZrtpCpp) that the sequence number is only intended for diagnostic purposes and not for enforcing any kind of packet ordering. 

ortp_zrtp_sequence_patch.txt
Description: Text document