80 lines
2.3 KiB
Go
80 lines
2.3 KiB
Go
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package protocol
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// A PacketNumber in QUIC
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type PacketNumber int64
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// InvalidPacketNumber is a packet number that is never sent.
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// In QUIC, 0 is a valid packet number.
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const InvalidPacketNumber PacketNumber = -1
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// PacketNumberLen is the length of the packet number in bytes
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type PacketNumberLen uint8
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const (
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// PacketNumberLen1 is a packet number length of 1 byte
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PacketNumberLen1 PacketNumberLen = 1
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// PacketNumberLen2 is a packet number length of 2 bytes
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PacketNumberLen2 PacketNumberLen = 2
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// PacketNumberLen3 is a packet number length of 3 bytes
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PacketNumberLen3 PacketNumberLen = 3
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// PacketNumberLen4 is a packet number length of 4 bytes
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PacketNumberLen4 PacketNumberLen = 4
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)
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// DecodePacketNumber calculates the packet number based on the received packet number, its length and the last seen packet number
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func DecodePacketNumber(
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packetNumberLength PacketNumberLen,
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lastPacketNumber PacketNumber,
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wirePacketNumber PacketNumber,
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) PacketNumber {
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var epochDelta PacketNumber
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switch packetNumberLength {
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case PacketNumberLen1:
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epochDelta = PacketNumber(1) << 8
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case PacketNumberLen2:
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epochDelta = PacketNumber(1) << 16
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case PacketNumberLen3:
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epochDelta = PacketNumber(1) << 24
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case PacketNumberLen4:
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epochDelta = PacketNumber(1) << 32
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}
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epoch := lastPacketNumber & ^(epochDelta - 1)
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var prevEpochBegin PacketNumber
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if epoch > epochDelta {
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prevEpochBegin = epoch - epochDelta
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}
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nextEpochBegin := epoch + epochDelta
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return closestTo(
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lastPacketNumber+1,
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epoch+wirePacketNumber,
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closestTo(lastPacketNumber+1, prevEpochBegin+wirePacketNumber, nextEpochBegin+wirePacketNumber),
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)
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}
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func closestTo(target, a, b PacketNumber) PacketNumber {
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if delta(target, a) < delta(target, b) {
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return a
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}
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return b
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}
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func delta(a, b PacketNumber) PacketNumber {
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if a < b {
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return b - a
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}
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return a - b
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}
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// GetPacketNumberLengthForHeader gets the length of the packet number for the public header
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// it never chooses a PacketNumberLen of 1 byte, since this is too short under certain circumstances
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func GetPacketNumberLengthForHeader(packetNumber, leastUnacked PacketNumber) PacketNumberLen {
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diff := uint64(packetNumber - leastUnacked)
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if diff < (1 << (16 - 1)) {
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return PacketNumberLen2
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}
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if diff < (1 << (24 - 1)) {
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return PacketNumberLen3
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}
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return PacketNumberLen4
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}
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