Appendix A. Pseudocode

Appendix A. Pseudocode

This appendix provides pseudocode for selected functions referenced throughout this document.

A.1 Sample Variable-Length Integer Decoding

uint64_t DecodeVariableLengthInteger(const uint8_t *data) {
    uint64_t v = data[0];
    uint8_t prefix = v >> 6;
    uint8_t length = 1 << prefix;

    v = v & 0x3f;
    while (--length > 0) {
        v = (v << 8) | *++data;
    }
    return v;
}

A.2 Sample Packet Number Encoding Algorithm

EncodePacketNumber(uint64_t full_pn, uint64_t largest_acked) {
    // The number of bits must be at least one more
    // than the base-2 logarithm of the number of contiguous
    // unacknowledged packet numbers, including the new packet.
    
    uint64_t num_unacked = full_pn - largest_acked;
    uint8_t min_bits = log2(num_unacked) + 1;
    uint8_t num_bytes = (min_bits + 7) / 8;

    return TruncatePacketNumber(full_pn, num_bytes);
}

A.3 Sample Packet Number Decoding Algorithm

DecodePacketNumber(uint64_t largest_pn, uint64_t truncated_pn,
                   uint8_t pn_nbits) {
    uint64_t expected_pn = largest_pn + 1;
    uint64_t pn_win = 1ULL << pn_nbits;
    uint64_t pn_hwin = pn_win / 2;
    uint64_t pn_mask = pn_win - 1;

    // The incoming packet number should be greater than
    // expected_pn - pn_hwin and less than or equal to
    // expected_pn + pn_hwin
    
    uint64_t candidate_pn = (expected_pn & ~pn_mask) | truncated_pn;
    if (candidate_pn <= expected_pn - pn_hwin &&
        candidate_pn < (1ULL << 62) - pn_win) {
        return candidate_pn + pn_win;
    }
    if (candidate_pn > expected_pn + pn_hwin &&
        candidate_pn >= pn_win) {
        return candidate_pn - pn_win;
    }
    return candidate_pn;
}

A.4 Sample ECN Validation Algorithm

ValidateECNCounts(newest_acked, ECT0_increase,
                  ECT1_increase, CE_increase) {
    if (ECT0_increase + ECT1_increase + CE_increase <
        newly_acked_bytes) {
        // ECN validation failed
        DisableECN();
    }
}