void L2LAPDm::sendUFrameUA(const L2Frame& frame)
{
// Send UA frame with a echoed payload.
// This is used in the contention resolution procedure.
// GSM 04.06 5.4.1.4.
// The caller need not hold mLock.
OBJLOG(INFO) << "state=" << mState << " " << frame;
L2Address address(mR,mSAPI);
L2Control control(L2Control::UFormat,frame.PF(),0x0C);
L2Length length(frame.L());
L2Header header(address,control,length);
writeL1NoAck(L2Frame(header,frame.L3Part()));
}
void L2LAPDm::bufferIFrameData(const L2Frame& frame)
{
// Concatenate I-frames to form the L3 frame.
/*
GSM 04.06 5.5.2 states:
When a data link layer entity is not in an own receiver busy condition
and receives a valid I frame whose send sequence number is equal to the
current receive state variable V(R), the data link layer entity shall:
- if the M bit is set to "0", concatenate it with previously
received frames with the M bit set to "1", if any, and pass the complete
layer 3 message unit to the layer 3 entity using the primitive
DL-DATA-INDICATION;
- if the M bit is set to "1", store the information field of the frame and
concatenate it with previously received frames with the M bit set to "1",
if any (Note: no information is passed to the layer 3 entity);
*/
OBJLOG(DEBUG) << frame;
if (!frame.M()) {
// The last or only frame.
if (mRecvBuffer.size()==0) {
// The only frame -- just send it up.
OBJLOG(DEBUG) << "single frame message";
mL3Out.write(new L3Frame(frame));
return;
}
// The last of several -- concat and send it up.
OBJLOG(DEBUG) << "last frame of message";
mL3Out.write(new L3Frame(mRecvBuffer,frame.L3Part()));
mRecvBuffer.clear();
return;
}
// One segment of many -- concat.
// This is inefficient but simple.
mRecvBuffer = L3Frame(mRecvBuffer,frame.L3Part());
OBJLOG(DEBUG) <<"buffering recvBuffer=" << mRecvBuffer;
}
void L2LAPDm::receiveUFrameSABM(const L2Frame& frame)
{
// Caller should hold mLock.
// Process the incoming SABM command.
// GSM 04.06 3.8.2, 5.4.1
// Q.921 5.5.1.2.
// Also borrows from vISDN datalink.c:lapd_socket_handle_uframe_sabm.
OBJLOG(INFO) << "state=" << mState << " " << frame;
// Ignore frame if P!=1.
// See GSM 04.06 5.4.1.2.
if (!frame.PF()) return;
// Dispatch according to current state.
// BTW, LAPDm can always enter multiframe mode when requested,
// so that's another big simplification over ISDN/LAPD.
switch (mState) {
case LinkReleased:
// GSM 04.06 5.4.5, 5.4.1.2, 5.4.1.4
clearCounters();
mEstablishmentInProgress = true;
// Tell L3 what happened.
mL3Out.write(new L3Frame(ESTABLISH));
if (frame.L()) {
// Presence of an L3 payload indicates contention resolution.
// GSM 04.06 5.4.1.4.
mState=ContentionResolution;
mContentionCheck = frame.sum();
mL3Out.write(new L3Frame(frame.L3Part(),DATA));
// Echo back payload.
sendUFrameUA(frame);
} else {
mState=LinkEstablished;
sendUFrameUA(frame.PF());
}
break;
case ContentionResolution:
// GSM 04.06 5.4.1.4
// This guards against the remote possibility that two handsets
// are sending on the same channel at the same time.
// vISDN's LAPD doesn't need/do this since peers are hard-wired
if (frame.sum()!=mContentionCheck) break;
mState=LinkEstablished;
sendUFrameUA(frame);
break;
case AwaitingEstablish:
// Huh? This would mean both sides sent SABM at the same time/
// That should not happen in GSM.
sendUFrameUA(frame.PF());
OBJLOG(WARN) << "simulatenous SABM attempts";
break;
case AwaitingRelease:
// If we are awaiting release, we will not enter ABM.
// So we send DM to indicate that.
sendUFrameDM(frame.PF());
break;
case LinkEstablished:
// Latency in contention resolution, GSM 04.06 5.4.2.1.
if (mEstablishmentInProgress) {
if (frame.L()) sendUFrameUA(frame);
else sendUFrameUA(frame.PF());
break;
}
if (frame.L()) {
abnormalRelease();
break;
}
// Re-establishment procedure, GSM 04.06 5.6.3.
// This basically resets the ack engine.
// We should not actually see this, as of rev 2.4.
OBJLOG(WARN) << "reestablishment not really supported";
sendUFrameUA(frame.PF());
clearCounters();
break;
default:
unexpectedMessage();
return;
}
}