/// <summary>
/// Updates the sender state for the gap reports received in a SACH chunk from the
/// remote peer.
/// </summary>
/// <param name="sackGapBlocks">The gap reports from the remote peer.</param>
/// <param name="maxTSNDistance">The maximum distance any valid TSN should be from the current
/// ACK'ed TSN. If this distance gets exceeded by a gap report then it's likely something has been
/// miscalculated.</param>
private void ProcessGapReports(List <SctpTsnGapBlock> sackGapBlocks, uint maxTSNDistance)
{
uint lastAckTSN = _cumulativeAckTSN;
foreach (var gapBlock in sackGapBlocks)
{
uint goodTSNStart = _cumulativeAckTSN + gapBlock.Start;
if (SctpDataReceiver.GetDistance(lastAckTSN, goodTSNStart) > maxTSNDistance)
{
logger.LogWarning($"SCTP SACK gap report had a start TSN of {goodTSNStart} too distant from last good TSN {lastAckTSN}, ignoring rest of SACK.");
break;
}
else if (!SctpDataReceiver.IsNewer(lastAckTSN, goodTSNStart))
{
logger.LogWarning($"SCTP SACK gap report had a start TSN of {goodTSNStart} behind last good TSN {lastAckTSN}, ignoring rest of SACK.");
break;
}
else
{
uint missingTSN = lastAckTSN + 1;
logger.LogTrace($"SCTP SACK gap report start TSN {goodTSNStart} gap report end TSN {_cumulativeAckTSN + gapBlock.End} " +
$"first missing TSN {missingTSN}.");
while (missingTSN != goodTSNStart)
{
if (!_missingChunks.ContainsKey(missingTSN))
{
if (!_unconfirmedChunks.ContainsKey(missingTSN))
{
// What to do? Can't retransmit a chunk that's no longer available.
// Hope it's a transient error from a duplicate or out of order SACK.
// TODO: Maybe keep count of how many time this occurs and send an ABORT if it
// gets to a certain threshold.
logger.LogWarning($"SCTP SACK gap report reported missing TSN of {missingTSN} but no matching unconfirmed chunk available.");
break;
}
else
{
logger.LogTrace($"SCTP SACK gap adding retransmit entry for TSN {missingTSN}.");
_missingChunks.TryAdd(missingTSN, 0);
}
}
missingTSN++;
}
}
lastAckTSN = _cumulativeAckTSN + gapBlock.End;
}
}
/// <summary>
/// Handler for SACK chunks received from the remote peer.
/// </summary>
/// <param name="sack">The SACK chunk from the remote peer.</param>
public void GotSack(SctpSackChunk sack)
{
if (sack != null)
{
_inRetransmitMode = false;
unchecked
{
uint maxTSNDistance = SctpDataReceiver.GetDistance(_cumulativeAckTSN, TSN);
bool processGapReports = true;
if (_unconfirmedChunks.TryGetValue(sack.CumulativeTsnAck, out var result))
{
_lastAckedDataChunkSize = result.UserData.Length;
}
if (!_gotFirstSACK)
{
if (SctpDataReceiver.GetDistance(_initialTSN, sack.CumulativeTsnAck) < maxTSNDistance &&
SctpDataReceiver.IsNewerOrEqual(_initialTSN, sack.CumulativeTsnAck))
{
logger.LogTrace($"SCTP first SACK remote peer TSN ACK {sack.CumulativeTsnAck} next sender TSN {TSN}, arwnd {sack.ARwnd} (gap reports {sack.GapAckBlocks.Count}).");
_gotFirstSACK = true;
_cumulativeAckTSN = _initialTSN;
RemoveAckedUnconfirmedChunks(sack.CumulativeTsnAck);
}
}
else
{
if (_cumulativeAckTSN != sack.CumulativeTsnAck)
{
if (SctpDataReceiver.GetDistance(_cumulativeAckTSN, sack.CumulativeTsnAck) > maxTSNDistance)
{
logger.LogWarning($"SCTP SACK TSN from remote peer of {sack.CumulativeTsnAck} was too distant from the expected {_cumulativeAckTSN}, ignoring.");
processGapReports = false;
}
else if (!SctpDataReceiver.IsNewer(_cumulativeAckTSN, sack.CumulativeTsnAck))
{
logger.LogWarning($"SCTP SACK TSN from remote peer of {sack.CumulativeTsnAck} was behind expected {_cumulativeAckTSN}, ignoring.");
processGapReports = false;
}
else
{
logger.LogTrace($"SCTP SACK remote peer TSN ACK {sack.CumulativeTsnAck}, next sender TSN {TSN}, arwnd {sack.ARwnd} (gap reports {sack.GapAckBlocks.Count}).");
RemoveAckedUnconfirmedChunks(sack.CumulativeTsnAck);
}
}
else
{
logger.LogTrace($"SCTP SACK remote peer TSN ACK no change {_cumulativeAckTSN}, next sender TSN {TSN}, arwnd {sack.ARwnd} (gap reports {sack.GapAckBlocks.Count}).");
RemoveAckedUnconfirmedChunks(sack.CumulativeTsnAck);
}
}
// Check gap reports. Only process them if the cumulative ACK TSN was acceptable.
if (processGapReports && sack.GapAckBlocks.Count > 0)
{
ProcessGapReports(sack.GapAckBlocks, maxTSNDistance);
}
}
_receiverWindow = CalculateReceiverWindow(sack.ARwnd);
_congestionWindow = CalculateCongestionWindow(_lastAckedDataChunkSize);
// SACK's will normally allow more data to be sent.
_senderMre.Set();
}
}