packet_number.go

     1  // Copyright 2023 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package quic
     6  
     7  // A packetNumber is a QUIC packet number.
     8  // Packet numbers are integers in the range [0, 2^62-1].
     9  //
    10  // https://www.rfc-editor.org/rfc/rfc9000.html#section-12.3
    11  type packetNumber int64
    12  
    13  const maxPacketNumber = 1<<62 - 1 // https://www.rfc-editor.org/rfc/rfc9000.html#section-17.1-1
    14  
    15  // decodePacketNumber decodes a truncated packet number, given
    16  // the largest acknowledged packet number in this number space,
    17  // the truncated number received in a packet, and the size of the
    18  // number received in bytes.
    19  //
    20  // https://www.rfc-editor.org/rfc/rfc9000.html#section-17.1
    21  // https://www.rfc-editor.org/rfc/rfc9000.html#section-a.3
    22  func decodePacketNumber(largest, truncated packetNumber, numLenInBytes int) packetNumber {
    23  	expected := largest + 1
    24  	win := packetNumber(1) << (uint(numLenInBytes) * 8)
    25  	hwin := win / 2
    26  	mask := win - 1
    27  	candidate := (expected &^ mask) | truncated
    28  	if candidate <= expected-hwin && candidate < (1<<62)-win {
    29  		return candidate + win
    30  	}
    31  	if candidate > expected+hwin && candidate >= win {
    32  		return candidate - win
    33  	}
    34  	return candidate
    35  }
    36  
    37  // appendPacketNumber appends an encoded packet number to b.
    38  // The packet number must be larger than the largest acknowledged packet number.
    39  // When no packets have been acknowledged yet, largestAck is -1.
    40  //
    41  // https://www.rfc-editor.org/rfc/rfc9000.html#section-17.1-5
    42  func appendPacketNumber(b []byte, pnum, largestAck packetNumber) []byte {
    43  	switch packetNumberLength(pnum, largestAck) {
    44  	case 1:
    45  		return append(b, byte(pnum))
    46  	case 2:
    47  		return append(b, byte(pnum>>8), byte(pnum))
    48  	case 3:
    49  		return append(b, byte(pnum>>16), byte(pnum>>8), byte(pnum))
    50  	default:
    51  		return append(b, byte(pnum>>24), byte(pnum>>16), byte(pnum>>8), byte(pnum))
    52  	}
    53  }
    54  
    55  // packetNumberLength returns the minimum length, in bytes, needed to encode
    56  // a packet number given the largest acknowledged packet number.
    57  // The packet number must be larger than the largest acknowledged packet number.
    58  //
    59  // https://www.rfc-editor.org/rfc/rfc9000.html#section-17.1-5
    60  func packetNumberLength(pnum, largestAck packetNumber) int {
    61  	d := pnum - largestAck
    62  	switch {
    63  	case d < 0x80:
    64  		return 1
    65  	case d < 0x8000:
    66  		return 2
    67  	case d < 0x800000:
    68  		return 3
    69  	default:
    70  		return 4
    71  	}
    72  }
    73
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