RTP RR及SR包中NTP的转换

背景

在RTCP的RR及SR中report block存在一个ntp字段，这个值为RR或SR产生时间戳。RFC3550的第4节描述了它的意义

Wallclock time (absolute date and time) is represented using the
timestamp format of the Network Time Protocol (NTP), which is in
seconds relative to 0h UTC on 1 January 1900 [4]. The full
resolution NTP timestamp is a 64-bit unsigned fixed-point number with
the integer part in the first 32 bits and the fractional part in the
last 32 bits.

NTP的要点

首先明确:

• NTP时间戳有64位，其中32位表示second，32位表示 fraction second（就是秒后面的单位)
• 表示的秒数起点为 0h UTC on 1 January 1900 (不同于Unix时间的起始于1970)，fraction的单位为2^32 second

转换

明白了基本原理，就知道了怎么转换，首先需要定义两个常量，如下:

const uint32_t UnixNtpOffset = 0x83AA7E80 };
const uint64_t NtpFractionalUnit = { 1LL << 32 };

UnixNtpOffset 表示0h UTC on 1 January 1900 到 0h UTC on 1 January 1970的秒数
NtpFractionalUnit 表示fraction second 单位

那么NTP可以定义如下：

struct Ntp
{
    uint32_t seconds;
    uint32_t fractions;
};

下面的代码描述如何转换(gettimeofday的struct timeval与NTP相互转换)

void ConvertUnixTimeIntoNtpTime(struct timeval *unix, Ntp *ntp)
{
    ntp->seconds = unix->tv_sec + UnixNtpOffset;
    ntp->fractions = (uint32_t)( (double)(unix->tv_usec) * (double)(NtpFractionalUnit) * 1.0e-6 );
}

void ConvertNtpTimeIntoUnixTime(Ntp*ntp, struct timeval *unix)
{
    unix->tv_sec = ntp->seconds - UnixNtpOffset; 
    unix->tv_usec = (uint32_t)( (double)ntp->fractions * 1.0e6 / (double)(NtpFractionalUnit) );
}

struct timeval的tv_usev的单位为微妙，在转换为NTP的fraction时，先转换成秒。

封装

毫秒与NTP互转

class Time
{
    // Seconds from Jan 1, 1900 to Jan 1, 1970.
    static constexpr uint32_t UnixNtpOffset{ 0x83AA7E80 };
    // NTP fractional unit.
    static constexpr uint64_t NtpFractionalUnit{ 1LL << 32 };

    public:
        struct Ntp
        {
            uint32_t seconds;
            uint32_t fractions;
        };

        static Time::Ntp TimeMs2Ntp(uint64_t ms);
        static uint64_t Ntp2TimeMs(Time::Ntp ntp);
};

Time::Ntp Time::TimeMs2Ntp(uint64_t ms)
{
    Time::Ntp ntp; 

    ntp.seconds = ms / 1000 + UnixNtpOffset;
    ntp.fractions = static_cast<uint32_t>((static_cast<double>(ms % 1000) / 1000) * NtpFractionalUnit);
    return ntp;
 }

uint64_t Time::Ntp2TimeMs(Time::Ntp ntp)
{
    return (
                        static_cast<uint64_t>(ntp.seconds - UnixNtpOffset) * 1000 +
                        static_cast<uint64_t>(std::round((static_cast<double>(ntp.fractions) * 1000) / NtpFractionalUnit))
                );
}

参考:
https://*.com/questions/2641954/create-ntp-time-stamp-from-gettimeofday