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package protocol
// A PacketNumber in QUIC
type PacketNumber int64
// InvalidPacketNumber is a packet number that is never sent.
// In QUIC, 0 is a valid packet number.
const InvalidPacketNumber PacketNumber = -1
// PacketNumberLen is the length of the packet number in bytes
type PacketNumberLen uint8
const (
// PacketNumberLen1 is a packet number length of 1 byte
PacketNumberLen1 PacketNumberLen = 1
// PacketNumberLen2 is a packet number length of 2 bytes
PacketNumberLen2 PacketNumberLen = 2
// PacketNumberLen3 is a packet number length of 3 bytes
PacketNumberLen3 PacketNumberLen = 3
// PacketNumberLen4 is a packet number length of 4 bytes
PacketNumberLen4 PacketNumberLen = 4
)
// DecodePacketNumber calculates the packet number based its length and the last seen packet number
// This function is taken from https://www.rfc-editor.org/rfc/rfc9000.html#section-a.3.
func DecodePacketNumber(length PacketNumberLen, largest PacketNumber, truncated PacketNumber) PacketNumber {
expected := largest + 1
win := PacketNumber(1 << (length * 8))
hwin := win / 2
mask := win - 1
candidate := (expected & ^mask) | truncated
if candidate <= expected-hwin && candidate < 1<<62-win {
return candidate + win
}
if candidate > expected+hwin && candidate >= win {
return candidate - win
}
return candidate
}
// PacketNumberLengthForHeader gets the length of the packet number for the public header
// it never chooses a PacketNumberLen of 1 byte, since this is too short under certain circumstances
func PacketNumberLengthForHeader(pn, largestAcked PacketNumber) PacketNumberLen {
var numUnacked PacketNumber
if largestAcked == InvalidPacketNumber {
numUnacked = pn + 1
} else {
numUnacked = pn - largestAcked
}
if numUnacked < 1<<(16-1) {
return PacketNumberLen2
}
if numUnacked < 1<<(24-1) {
return PacketNumberLen3
}
return PacketNumberLen4
}
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