UDP how do Mumble handle packets out of order ?

[robertnax]

I am wondering if anyone can help me explain or point me to the code where Mumble handles out of order UDP packets ? i can see there is a sequence number but how do Mumble out of order packets is there a mechanism that will accumulate packets and sort them before it starts playing the audio ?

If anyone can help me with an explanation i would be happy and/or direct me to the code how it handles it it would be great as i would like to understand how 'reliable' i can assume it can be over UDP.

[Krzmbrzl]

I can't give you a full answer to your question since I never looked into that part of the code. I can however point you towards some sections in the code.

This is were UDP voice packets are received (notice the iSeq variable which is probably the one you want to trace):

mumble/src/mumble/ServerHandler.cpp

Lines 251 to 266 in a559839

	void ServerHandler::handleVoicePacket(unsigned int msgFlags, PacketDataStream &pds,
	MessageHandler::UDPMessageType type) {
	unsigned int uiSession;
	pds >> uiSession;
	ClientUser *p = ClientUser::get(uiSession);
	AudioOutputPtr ao = g.ao;
	if (ao && p && !(((msgFlags & 0x1f) == 2) && g.s.bWhisperFriends && p->qsFriendName.isEmpty())) {
	unsigned int iSeq;
	pds >> iSeq;
	QByteArray qba;
	qba.reserve(pds.left() + 1);
	qba.append(static_cast< char >(msgFlags));
	qba.append(pds.dataBlock(pds.left()));
	ao->addFrameToBuffer(p, qba, iSeq, type);
	}
	}

The packet is given to the AudioOutput class

mumble/src/mumble/AudioOutput.cpp

Line 184 in a559839

aop->addFrameToBuffer(qbaPacket, iSeq);

from where it is passed to AudioOutputSpeech where it is packed in a JitterBuffer

mumble/src/mumble/AudioOutputSpeech.cpp

Lines 216 to 222 in a559839

	JitterBufferPacket jbp;
	jbp.data = const_cast< char * >(qbaPacket.constData());
	jbp.len = qbaPacket.size();
	jbp.span = samples;
	jbp.timestamp = iFrameSize * iSeq;
	jitter_buffer_put(jbJitter, &jbp);

So as you can see the sequence number determines the time stamp of the audio. I simply assume that if the timestamp lies in the past, the package is basically discarded whereas if it lays in the future it will be placed there in the hope that the packet in between arrives in time to take its place 🤷