Byte Order, Alignment, and Time Format

   All integer fields are carried in network byte order, that is, most
   significant byte (octet) first.  This byte order is commonly known as
   big-endian.  The transmission order is described in detail in [3].
   Unless otherwise noted, numeric constants are in decimal (base 10).

   All header data is aligned to its natural length, i.e., 16-bit fields
   are aligned on even offsets, 32-bit fields are aligned at offsets
   divisible by four, etc.  Octets designated as padding have the value
   zero.

   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.  In some fields where a more compact representation is
   appropriate, only the middle 32 bits are used; that is, the low 16
   bits of the integer part and the high 16 bits of the fractional part.
   The high 16 bits of the integer part must be determined
   independently.

   An implementation is not required to run the Network Time Protocol in
   order to use RTP.  Other time sources, or none at all, may be used
   (see the description of the NTP timestamp field in Section 6.4.1).
   However, running NTP may be useful for synchronizing streams
   transmitted from separate hosts.

   The NTP timestamp will wrap around to zero some time in the year
   2036, but for RTP purposes, only differences between pairs of NTP
   timestamps are used.  So long as the pairs of timestamps can be
   assumed to be within 68 years of each other, using modular arithmetic
   for subtractions and comparisons makes the wraparound irrelevant.