DIRECTORY
ArpaIP USING [Buffers, FreeBuffers, GetUserBytes, RecvProc],
List USING [Nconc1],
ArpaTCP USING [DByte, Error],
ArpaTCPLogging USING [Direction, PrintMessageWithSrc, PrintStateChange, PrintTCPPacket],
ArpaTCPOps USING [ChecksumsMatch, Flip, pktsDuplicate, pktsFromFuture, pktsFromPast, pktsRcvd, pktsWithNoConnection, pktsWithBadChecksum, repacketizing, sendBufferLength, SetTimeout, TCPChecksum, TCPHandle, TCPHeaderP, TCPRcvBuffer, TimeWaitTimeoutFromHandle],
ArpaTCPReceiving USING [],
ArpaTCPStates USING [CloseConnection, FindHandle, GetInitialSequenceNumber],
ArpaTCPTransmit USING [RemoveAckedSegments, SendReset, SendSYN, TryToSend, TryToSendData];
ArpaTCPReceivingImpl: CEDAR MONITOR LOCKS handle USING handle: TCPHandle
IMPORTS ArpaIP, List, ArpaTCP, ArpaTCPLogging, ArpaTCPOps, ArpaTCPStates, ArpaTCPTransmit
EXPORTS ArpaTCPReceiving =
BEGIN OPEN ArpaTCPOps, ArpaTCPReceiving;
SeqCode: TYPE = {pastSeq, okSeq, futureSeq}; 
ProcessRcvdSegment: PUBLIC ArpaIP.RecvProc = TRUSTED {
rcvdDatagram: ArpaIP.Buffers _ b;
tcpHdrPtr: TCPHeaderP;
nSeqBytes: INT;
handle: TCPHandle;
checksum: ArpaTCP.DByte _ ArpaTCPOps.TCPChecksum[rcvdDatagram];

pktsRcvd _ pktsRcvd + 1; 
tcpHdrPtr _ LOOPHOLE[@rcvdDatagram.body];
IF ~ArpaTCPOps.ChecksumsMatch[checksum, tcpHdrPtr.checksum] THEN {
pktsWithBadChecksum _ pktsWithBadChecksum + 1; 
ArpaTCPLogging.PrintMessageWithSrc["Bad checksum", rcvdDatagram.hdr1.source]; 
RETURN; }; 

nSeqBytes _ ArpaIP.GetUserBytes[rcvdDatagram].bodyBytes - tcpHdrPtr.dataWordOffset * 4; 
IF tcpHdrPtr.syn THEN nSeqBytes _ nSeqBytes + 1; 
IF tcpHdrPtr.fin THEN nSeqBytes _ nSeqBytes + 1; 

handle _ ArpaTCPStates.FindHandle[rcvdDatagram]; 
ArpaTCPLogging.PrintTCPPacket[handle, rcvdDatagram, fromNet]; 
IF handle = NIL THEN {
IF ~tcpHdrPtr.rst THEN -- if no matching TCB
IF tcpHdrPtr.ack THEN -- if not a reset, then send a reset
ArpaTCPTransmit.SendReset[NIL, tcpHdrPtr.dstnPort, tcpHdrPtr.sourcePort, rcvdDatagram.hdr1.source, Flip[tcpHdrPtr.ackNumber], 0] -- with no ack 
ELSE
ArpaTCPTransmit.SendReset[NIL, tcpHdrPtr.dstnPort, tcpHdrPtr.sourcePort, rcvdDatagram.hdr1.source, 0, Flip[tcpHdrPtr.seqNumber] + nSeqBytes]; -- with ack 
pktsWithNoConnection _ pktsWithNoConnection + 1; 
ArpaTCPLogging.PrintMessageWithSrc["No connection", rcvdDatagram.hdr1.source]; 
RETURN; }; 
ContinueProcessingRcvdSegment[handle, rcvdDatagram]; 
};
ContinueProcessingRcvdSegment: ENTRY PROC [handle: TCPHandle, rcvdDatagram: ArpaIP.Buffers] = {
ENABLE UNWIND => NULL;
tcpRcvBufferPtr: REF TCPRcvBuffer; -- info block for rcvd packet 
tcpHdrPtr: TCPHeaderP; -- pointer to tcp header
ackNumber, seqNumber: INT; -- from tcpHdrPtr, Flipped for us
seqResult: SeqCode; -- result of seq number check 
nSeqBytes: INT; -- number of data bytes + syn, fin 
continueProcessing: BOOL; -- true if more processing to do 
freeSegment: BOOL; -- true to dispose of segment 
finFlag: BOOL; -- true if segment contains fin 
sendAck: BOOL; -- set to send ack 
sendData: BOOL; -- set to send data 
connectionClosed: BOOL; -- set when close connection so that its not referenced again 

DisposeRcvdBuffer: PROC = { -- dispose of pkts on fromNet queue 
IF handle.fromNetQueue # NIL THEN {
ArpaIP.FreeBuffers[tcpRcvBufferPtr.datagramPtr];
handle.fromNetQueue _ handle.fromNetQueue.rest; 
};
};

ProcessReset: PROC = {
SELECT handle.state FROM 
listen => freeSegment _ TRUE; -- ignore the reset 
synSent => TRUSTED { -- if segment has acceptable ack, close connection 
IF tcpHdrPtr.ack AND ackNumber-handle.iss > 0 AND ackNumber-handle.sndNxt <= 0 THEN {
ArpaTCPStates.CloseConnection[handle, remoteAbort]; 
connectionClosed _ TRUE; }; };
synRcvd => -- if were listening, return to listen else reset 
IF ~handle.active THEN {
ArpaTCPLogging.PrintStateChange[handle, listen];
handle.state _ listen;  } 
ELSE {
ArpaTCPStates.CloseConnection[handle, remoteAbort]; 
connectionClosed _ TRUE; };
established, finWait1, finWait2, closeWait, closing, lastAck, timeWait => {
ArpaTCPStates.CloseConnection[handle, remoteAbort]; 
connectionClosed _ TRUE; };
ENDCASE; }; 

SignalUserUrgent: INTERNAL PROC = {
BROADCAST handle.urgentAvailable; }; 

ProcessUrgent: INTERNAL PROC = TRUSTED {
IF ~handle.urgentMode THEN {
handle.urgentMode _ TRUE; 
handle.rcvUp _ tcpHdrPtr.urgentPtr + seqNumber; 
IF handle.state = established OR handle.state = finWait1 OR handle.state = finWait2 THEN
SignalUserUrgent[] } 
ELSE  -- else update urgent pointer 
IF handle.rcvUp < tcpHdrPtr.urgentPtr + seqNumber THEN 
handle.rcvUp _ tcpHdrPtr.urgentPtr + seqNumber; }; 

ProcessAck: INTERNAL PROC = {
nDataBytesAcked: INT;
SELECT handle.state FROM 
listen => {-- no acks are acceptable, send reset 
TRUSTED {ArpaTCPTransmit.SendReset[handle, handle.localPort, tcpHdrPtr.sourcePort, rcvdDatagram.hdr1.source, ackNumber, 0]};
freeSegment _ TRUE; 
continueProcessing _ FALSE; 
RETURN;
};
synRcvd, synSent => 
IF ackNumber - handle.iss <= 0 OR ackNumber - handle.sndNxt > 0 THEN { -- if unacceptable ack, then send reset
TRUSTED {ArpaTCPTransmit.SendReset[handle, handle.localPort, handle.foreignPort, handle.foreignAddr, ackNumber, 0]};
freeSegment _ TRUE; 
continueProcessing _ FALSE; 
RETURN;
}
ENDCASE; 
IF ackNumber - handle.sndNxt > 0 THEN { -- ack from future send an ack and discard the packet 
sendAck _ TRUE; 
freeSegment _ TRUE;
continueProcessing _ FALSE; 
RETURN;
}; 
IF (handle.sndWL1 - seqNumber < 0 OR handle.sndWL1 = seqNumber AND handle.sndWL2 - ackNumber <= 0) OR (handle.sndWL1 = 0 AND handle.sndWL2 = 0) THEN {
TRUSTED {handle.sndWnd _ MIN[tcpHdrPtr.window, ArpaTCPOps.sendBufferLength]};
handle.sndWL1 _ seqNumber; 
handle.sndWL2 _ ackNumber;
BROADCAST handle.windowAvailable; -- tell waiters that there may be some room now
}; 
IF ackNumber - handle.sndUna <= 0 THEN -- duplicate ack
RETURN; -- ignore ack
nDataBytesAcked _ ackNumber - handle.sndUna; 
IF handle.sndUna = handle.iss AND nDataBytesAcked # 0 THEN 
nDataBytesAcked _ nDataBytesAcked - 1; 
IF (SELECT handle.state FROM 
finWait1, finWait2, closing, lastAck, timeWait => TRUE
ENDCASE => FALSE) AND 
ackNumber - handle.finSequence >= 0 AND 
nDataBytesAcked # 0 THEN 
nDataBytesAcked _ nDataBytesAcked - 1; 
IF handle.nBytesToSend >= nDataBytesAcked THEN -- Occasional BoundsFault
handle.nBytesToSend _ handle.nBytesToSend - nDataBytesAcked; 
IF repacketizing THEN -- if repacketizing, update send ptr in TCP buffer 
handle.sendSlot _ (handle.sendSlot + nDataBytesAcked) MOD sendBufferLength; 

TRUSTED {handle.sndUna _ Flip[tcpHdrPtr.ackNumber]}; -- update ack info in handle 
ArpaTCPTransmit.RemoveAckedSegments[handle]; -- dispose of acked segments or return to user
BROADCAST handle.windowAvailable; -- tell waiters that there may be some room now
SELECT handle.state FROM 
synRcvd => {
ArpaTCPLogging.PrintStateChange[handle, established]; 
handle.state _ established; 
IF handle.urgentMode THEN -- if rcvd urgent when in synrcvd state
SignalUserUrgent[]; -- then tell user now 
sendData _ TRUE;
};
established => sendData _ TRUE;
finWait1 => 
IF ackNumber - handle.finSequence > 0 THEN {-- if our fin acked, then state is finwait2 
ArpaTCPLogging.PrintStateChange[handle, finWait2];
handle.state _ finWait2; 
};

closeWait => {
continueProcessing _ FALSE; 
freeSegment _ TRUE; 
sendData _ TRUE; -- try to send more data
};
closing => {
IF ackNumber > handle.finSequence THEN { -- if fin acked, state is timewait
ArpaTCPLogging.PrintStateChange[handle, timeWait]; 
handle.state _ timeWait; 
handle.timeWaitTime _ SetTimeout[TimeWaitTimeoutFromHandle[handle]];
}; 
continueProcessing _ FALSE; 
freeSegment _ TRUE;
};
lastAck => {
IF ackNumber > handle.finSequence THEN { -- close connection if fin acked
ArpaTCPStates.CloseConnection[handle, handle.reason];
connectionClosed _ TRUE
} 
ELSE 
freeSegment _ TRUE; 
continueProcessing _ FALSE
}
ENDCASE
}; -- ProcessAck 

ProcessSyn: INTERNAL PROC = {
SELECT handle.state FROM 
listen => {
ArpaTCPLogging.PrintStateChange[handle, synRcvd]; 
handle.state _ synRcvd;
handle.irs _ seqNumber; 
handle.rcvNxt _ seqNumber + 1; 
handle.iss _ ArpaTCPStates.GetInitialSequenceNumber[]; 
handle.sndUna _ handle.iss; 
BROADCAST handle.notListening;
handle.matchForeignAddr _ TRUE; 
handle.matchForeignPort _ TRUE; 
handle.foreignAddr _ tcpRcvBufferPtr.datagramPtr.hdr1.source; 
TRUSTED {handle.foreignPort _ tcpHdrPtr.sourcePort}; 
ArpaTCPTransmit.SendSYN[handle]; -- send a syn and ack 
tcpRcvBufferPtr.dataByteCount _ tcpRcvBufferPtr.dataByteCount - 1; 
tcpRcvBufferPtr.offsetSeqNo _ tcpRcvBufferPtr.offsetSeqNo + 1; 
TRUSTED {tcpHdrPtr.syn _ FALSE}; -- don't reprocess syn 
continueProcessing _ FALSE; -- don't process data or fin until in established state
};
synSent => {
handle.irs _ seqNumber; -- initial rcv sequence 
handle.rcvNxt _ seqNumber + 1; 
tcpRcvBufferPtr.dataByteCount _ tcpRcvBufferPtr.dataByteCount - 1; 
tcpRcvBufferPtr.offsetSeqNo _ tcpRcvBufferPtr.offsetSeqNo + 1; 
IF handle.sndUna > handle.iss THEN { -- if our syn was acked then conn. established 
ArpaTCPLogging.PrintStateChange[handle, established];
handle.state _ established; 
}
ELSE { -- if our syn not acked, state becomes synRcvd
ArpaTCPLogging.PrintStateChange[handle, synRcvd]; 
handle.state _ synRcvd; 
continueProcessing _ FALSE; -- wait till in established state
}; 
TRUSTED {tcpHdrPtr.syn _ FALSE}; -- don't reprocess syn 
sendAck _ TRUE; -- ack the syn received
};
synRcvd => {
ArpaTCPLogging.PrintStateChange[handle, listen]; 
handle.state _ listen; -- return to listen  (why? -DN)
continueProcessing _ FALSE; -- stop processing 
freeSegment _ TRUE; -- dispose of buffer
}
ENDCASE => { -- in any other state, reset connection 
ArpaTCPStates.CloseConnection[handle, protocolViolation]; 
connectionClosed _ TRUE;
}
}; -- ProcessSyn

ProcessFin: INTERNAL PROC = {
SELECT handle.state FROM 
synRcvd, established => {
ArpaTCPLogging.PrintStateChange[handle, closeWait];
handle.state _ closeWait;
handle.reason _ remoteClose;
};
finWait1 => {
ArpaTCPLogging.PrintStateChange[handle, closing];
handle.state _ closing;
};
finWait2 => {
ArpaTCPLogging.PrintStateChange[handle, timeWait];
handle.state _ timeWait; 
handle.timeWaitTime _ SetTimeout[TimeWaitTimeoutFromHandle[handle]];
};
ENDCASE;
BROADCAST handle.dataAvailable; -- tell the user that that data is never coming now.
}; -- ProcessFin
TRUSTED {
tcpHdrPtr _ LOOPHOLE[@rcvdDatagram.body];
ackNumber _ Flip[tcpHdrPtr.ackNumber];
seqNumber _ Flip[tcpHdrPtr.seqNumber];
nSeqBytes _ ArpaIP.GetUserBytes[rcvdDatagram].bodyBytes - tcpHdrPtr.dataWordOffset * 4; 
IF ~(handle.state = listen OR handle.state = synSent) THEN {
seqResult _ CheckSequenceNumber[seqNumber, nSeqBytes, handle.rcvNxt, handle.rcvWnd]; 
IF seqResult # okSeq THEN {
SELECT seqResult FROM
pastSeq => {
ArpaTCPLogging.PrintMessageWithSrc["Packet from past", handle.foreignAddr]; 
pktsFromPast _ pktsFromPast + 1; };
futureSeq => {
ArpaTCPLogging.PrintMessageWithSrc["Future packet", handle.foreignAddr]; 
pktsFromFuture _ pktsFromFuture + 1; }; 
ENDCASE => ERROR;
IF ~tcpHdrPtr.rst THEN ArpaTCPTransmit.TryToSend[handle]; -- ~reset => send an ack 
RETURN; }; }; 
IF tcpHdrPtr.syn THEN nSeqBytes _ nSeqBytes + 1; 
IF tcpHdrPtr.fin THEN nSeqBytes _ nSeqBytes + 1; 

continueProcessing _ TRUE; -- more info in packet to process 
freeSegment _ FALSE; -- don't dispose of packet 
sendAck _ FALSE; -- set to send an ack 
sendData _ FALSE; -- set to try to send more data 
connectionClosed _ FALSE; -- set if the connection closes

IF tcpHdrPtr.rst THEN { -- if its a reset, process it and exit 
ProcessReset[]; 
RETURN; };

IF tcpHdrPtr.psh THEN { -- if its a push, process it 
sendAck _ TRUE;  -- at least ack a push since it is legit to push zero bytes
};

IF tcpHdrPtr.ack THEN { -- if its an ack, process it 
ProcessAck[]; 
IF connectionClosed THEN RETURN; 
IF nSeqBytes = 0 AND handle.zeroRcvWnd THEN  -- received probe
sendAck _ TRUE;  -- this might reopen the window
IF freeSegment OR (nSeqBytes = 0 AND ~tcpHdrPtr.urg) OR ~continueProcessing THEN GOTO SendAndExit; }; 

IF tcpHdrPtr.urg THEN -- if in state where data is acceptable, process urgent
SELECT handle.state FROM 
synRcvd, established, finWait1, finWait2 => ProcessUrgent[];
ENDCASE => NULL; 

IF nSeqBytes > handle.rcvWnd THEN {
SIGNAL PacketTooBig; -- HGM want's to look at one
ArpaTCPLogging.PrintMessageWithSrc["Packet too big", handle.foreignAddr]; 
nSeqBytes _ handle.rcvWnd; };

tcpRcvBufferPtr _ NEW[TCPRcvBuffer]; -- set up receive info block 
tcpRcvBufferPtr.datagramPtr _ rcvdDatagram; 
tcpRcvBufferPtr.dataByteCount _ nSeqBytes; 
tcpRcvBufferPtr.dataOffset _ tcpHdrPtr.dataWordOffset * 4;
tcpRcvBufferPtr.offsetSeqNo _ Flip[tcpHdrPtr.seqNumber]; 
tcpRcvBufferPtr.tcpHdrPtr _ tcpHdrPtr; 
QueueRcvdSegment[handle, tcpRcvBufferPtr]; -- put on handle fromNet queue
DO
IF handle.fromNetQueue = NIL THEN GOTO SendAndExit;
tcpRcvBufferPtr _ NARROW[handle.fromNetQueue.first];
IF ~(handle.state = listen OR handle.state = synSent) THEN -- if in synced state and
IF tcpRcvBufferPtr.offsetSeqNo # handle.rcvNxt THEN -- dont have next seq
GOTO SendAndExit ; 

tcpHdrPtr _ tcpRcvBufferPtr.tcpHdrPtr;
ackNumber _ Flip[tcpHdrPtr.ackNumber];
seqNumber _ Flip[tcpHdrPtr.seqNumber];

IF tcpHdrPtr.syn THEN {
ProcessSyn[];
IF connectionClosed THEN RETURN; 
IF freeSegment OR tcpRcvBufferPtr.dataByteCount = 0 THEN {
DisposeRcvdBuffer[]; 
IF continueProcessing THEN -- dispose of processed or empty packets 
LOOP; -- get next packet to process
}; 
IF ~continueProcessing THEN GOTO SendAndExit; }; 
IF handle.state = listen OR handle.state = synSent THEN {-- can't do anything more in these states 
DisposeRcvdBuffer[]; 
GOTO SendAndExit; }; 
IF ~tcpHdrPtr.ack THEN { -- ignore segments without acks  (why? -DN)
DisposeRcvdBuffer[]; 
LOOP; }; -- get next packet to process

handle.rcvNxt _ handle.rcvNxt + tcpRcvBufferPtr.dataByteCount; 
IF tcpHdrPtr.fin THEN { -- remember if fin is set as buffer is sent to user 
finFlag _ TRUE; 
tcpRcvBufferPtr.dataByteCount _ tcpRcvBufferPtr.dataByteCount - 1; }
ELSE finFlag _ FALSE; 
IF tcpRcvBufferPtr.dataByteCount > 0 THEN { -- if there's data then send it to the user
handle.fromNetQueue _ handle.fromNetQueue.rest;
SendDataToUser[handle, tcpRcvBufferPtr]; -- send to user and update window
sendAck _ TRUE; } -- remember to send an ack
ELSE { 
DisposeRcvdBuffer[]; -- if no data dispose of buffer
IF finFlag THEN sendAck _ TRUE; }; 

IF finFlag THEN ProcessFin[];

ENDLOOP;

EXITS -- repeat processing each segment received 
SendAndExit => 
SELECT TRUE FROM
sendAck => ArpaTCPTransmit.TryToSend[handle ! ArpaTCP.Error => CONTINUE] ;-- send ack and data
sendData => ArpaTCPTransmit.TryToSendData[handle ! ArpaTCP.Error => CONTINUE]; -- send only if data to send 
ENDCASE; 
};
}; -- ProcessRcvdSegment


CheckSequenceNumber: INTERNAL PROC [seq, segLen, rcvNxt, rcvWnd: INT] RETURNS [result: SeqCode] = {
rcvEnd: INT _ rcvWnd + rcvNxt; -- rcvwnd is rcvNxt to rcvEnd-1 inclusive
segEnd: INT _ seq + segLen - 1; -- segment is seq to segEnd inclusive 
IF segLen = 0 AND rcvWnd = 0 THEN
IF seq = rcvNxt THEN result _ okSeq 
ELSE 
IF seq - rcvNxt > 0 THEN result _ futureSeq 
ELSE result _ pastSeq 
ELSE 
IF segLen = 0 AND rcvWnd > 0 THEN 
IF seq - rcvNxt < 0 THEN result _ pastSeq 
ELSE 
IF seq - rcvEnd >= 0 THEN result _ futureSeq 
ELSE result _ okSeq 
ELSE 
IF rcvWnd = 0 THEN
IF seq >= rcvNxt THEN result _ futureSeq 
ELSE result _ pastSeq 
ELSE 
IF segEnd < rcvNxt THEN result _ pastSeq 
ELSE 
IF seq >= rcvEnd THEN result _ futureSeq 
ELSE  result _ okSeq
}; -- CheckSequenceNumber

 
QueueRcvdSegment: INTERNAL PROC [handle: TCPHandle, newBufferPtr: REF TCPRcvBuffer] = {
newStart: INT; -- first seq number in new segment 
newEnd: INT; -- last seq number in new segment 
oldStart: INT; -- first seq number in segment on queue 
oldEnd: INT; -- last seq number in segment on queue 
oldBufferPtr: REF TCPRcvBuffer; -- buffer on from net queue
l, prevL: LIST OF REF ANY; -- point to the cons cells as we walk the list
IF ~(handle.state = listen OR handle.state = synSent) THEN -- if in synced state 
IF newBufferPtr.offsetSeqNo < handle.rcvNxt THEN {
newBufferPtr.dataByteCount _ newBufferPtr.dataByteCount - (handle.rcvNxt - newBufferPtr.offsetSeqNo); 
newBufferPtr.dataOffset _ newBufferPtr.dataOffset + (handle.rcvNxt - newBufferPtr.offsetSeqNo); 
newBufferPtr.offsetSeqNo _ handle.rcvNxt; }; 
IF handle.fromNetQueue = NIL THEN {
handle.fromNetQueue _ LIST[newBufferPtr];
RETURN; }; 
prevL _ NIL; 
l _ handle.fromNetQueue;
newStart _ newBufferPtr.offsetSeqNo; 
newEnd _ newStart + newBufferPtr.dataByteCount - 1; 
WHILE l # NIL DO
oldBufferPtr _ NARROW[l.first];
oldStart _ oldBufferPtr.offsetSeqNo; 
oldEnd _ oldStart + oldBufferPtr.dataByteCount - 1; 
SELECT TRUE FROM
newStart < oldStart AND newEnd < oldStart => { -- new segment is before old one
IF prevL = NIL THEN
handle.fromNetQueue _ CONS[newBufferPtr, handle.fromNetQueue]
ELSE prevL.rest _ CONS[newBufferPtr, prevL.rest];
RETURN; }; 
newStart > oldEnd => NULL; -- new segment is after old one, keep going
newStart >= oldStart AND newEnd <= oldEnd => {
pktsDuplicate _ pktsDuplicate + 1;
handle.duplicates _ handle.duplicates + 1;
ArpaIP.FreeBuffers[newBufferPtr.datagramPtr];
RETURN; }; 
newStart <= oldStart AND newEnd >= oldEnd => {
pktsDuplicate _ pktsDuplicate + 1;
handle.duplicates _ handle.duplicates + 1;
ArpaIP.FreeBuffers[oldBufferPtr.datagramPtr];
oldBufferPtr.datagramPtr _ NIL;
IF prevL = NIL THEN handle.fromNetQueue _ l _ l.rest
ELSE prevL.rest _ l _ l.rest;
IF l = NIL THEN { -- if queue is now empty, just enqueue new buffer
IF prevL = NIL THEN handle.fromNetQueue _ LIST[newBufferPtr]
ELSE prevL.rest _ LIST[newBufferPtr];
RETURN; }
ELSE LOOP; }; -- else continue searching queue
newEnd < oldEnd => { -- tail of new overlaps old 
newBufferPtr.dataByteCount _ oldStart - newStart; 
IF prevL = NIL THEN
handle.fromNetQueue _ CONS[newBufferPtr, handle.fromNetQueue]
ELSE prevL.rest _ CONS[newBufferPtr, prevL.rest];
RETURN };
ENDCASE => { -- head of new overlaps old
newStart _ oldEnd + 1; 
newBufferPtr.offsetSeqNo _ newStart; 
newBufferPtr.dataByteCount _ newEnd - oldEnd;
newBufferPtr.dataOffset _ newBufferPtr.dataOffset + oldEnd - newStart + 1; };
prevL _ l; 
l _ l.rest;
ENDLOOP; -- check for more overlapping segments -- 
IF prevL = NIL THEN handle.fromNetQueue _ LIST[newBufferPtr]
ELSE prevL.rest _ LIST[newBufferPtr]; }; -- QueueRcvdSegment 
PacketTooBig: SIGNAL = CODE;
NegativeWindow: SIGNAL = CODE;
SendDataToUser: INTERNAL PROC [handle: ArpaTCPOps.TCPHandle, tcpRcvBufferPtr: REF TCPRcvBuffer] = TRUSTED  {
IF tcpRcvBufferPtr.tcpHdrPtr.urg THEN {
tcpRcvBufferPtr.urg _ TRUE; 
tcpRcvBufferPtr.endUrgentData _ handle.rcvUp - tcpRcvBufferPtr.offsetSeqNo; 
handle.urgentMode _ FALSE} 
ELSE  tcpRcvBufferPtr.urg _ FALSE; 

IF tcpRcvBufferPtr.dataByteCount > handle.rcvWnd THEN {
SIGNAL PacketTooBig; -- HGM want's to look at one
ArpaTCPLogging.PrintMessageWithSrc["Packet too big.", handle.foreignAddr]; 
tcpRcvBufferPtr.dataByteCount _ handle.rcvWnd; };

handle.rcvWnd _ handle.rcvWnd - tcpRcvBufferPtr.dataByteCount; 
IF handle.rcvWnd < 0 THEN SIGNAL NegativeWindow;
handle.readyToReadQueue _ List.Nconc1[handle.readyToReadQueue, tcpRcvBufferPtr];
BROADCAST handle.dataAvailable; }; 
END.
4ArpaTCPReceivingImpl.mesa
Copyright (C) 1983, 1985 by Xerox Corporation. All rights reserved. The following program was created in 1983 but has not been published within the meaning of the copyright law, is furnished under license, and may not be used, copied and/or disclosed except in accordance with the terms of said license.
Last Edited by: Nichols, September 1, 1983 4:25 pm
Last Edited by: Taft, January 4, 1984 11:13 am
Last Edited by: HGM, February 23, 1985 11:32:59 pm PST
Demers, September 7, 1988 1:48:42 pm PDT
Carl Hauser, April 22, 1988 11:54:16 am PDT
Doug Terry, April 21, 1988 1:05:18 pm PDT
Hal Murray July 16, 1985 4:52:04 pm PDT
John Larson, June 16, 1986 9:10:34 pm PDT
[b: Buffers] RETURNS [rB: Buffers]
This procedure does the processing of a datagram received from the network:
It checks the checksum and if it is bad disposes of the packet received.
It checks for a matching handle and if none is found, it sends a reset and disposes of the packet received.
If it finds a handle, it calls ContinueProcessingRcvdSegment to process the segment with the handle lock held.
check checksum 
find matching connection 
This procedure finishes the processing of a datagram received from the network:
It checks the sequence number and if it is outside the window,
it sends an ack and disposes of the packet received.
It processes resets; if the packet contains a reset, processing is finished.
It processes acks.
It processes urgents.
It queues the segment on the TCB FromNet queue in sequence number order.
It processes packets on the FromNet Queue until the next expected sequence number is not present:
It processes SYNs.
It processes data and sends it to the user.
It processes FINs.
not in urgent mode yet;  set urgent ptr and signal user 
acceptable ack
if packet has new sequence number or most recent sequence number and new ack then update the window (also update the window if we've never set it before.  It it fairly unlikely that sndWL1 and sndWL2 will both go to zero at the same time and even if they do it won't cause much harm.).
calculate number of data bytes acked, i.e. not including SYN and FIN, and number of unacked bytes to send or rexmit 
no data expected in following states, finished processing 

begin ContinueProcessingRcvdSegment 
initialize flags used in processing segment 
What else shoud we do with a push?
SU-AI used to do something that provoked a problem way downstream.
Truncating is slightly bogus if SendDataToUser is going to chop off part of the front
put segment in sequence number order on queue of segments to be processed 
process SYN, data, and FIN, repeat until FromNetQueue is empty or next expected sequence number has not been received 
process SYN
process data, send buffer to user, dispose of buffer if no data 
process fin 
Determine whether the packet is from the past or the future or whether it is in the current window (from next expected sequence number to next expected sequence number + window). 
Called when packet received from net contains SYN, FIN or data and has an acceptable sequence number (within the receive window).  Packets are queued in sequence number order on the TCB FromNet queue.  If packets overlap, then the DataPtr and DataByteCount fields in the TCPRcvBuffer are adjusted to "trim" the packet to contain only the non-overlapping data.  Packets that are duplicates are discarded. 
buffer overlaps start of window trim buffer to start of window 
queue is empty => put new segment at head 
new segment is duplicate 
old segment is duplicate discard old segment 
use first part of new segment 
continue with last part of new segment 
Debugging crap
Queues the datagram on the readyToRead queue and lets any waiting processes know that it's there.
LLL-MFE used to do something that provoked a problem here.
Truncating is slightly bogus if SendDataToUser is going to chop off part of the front
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