IPv4

Type of service

Type of Service: 8 bits

   The Type of Service provides an indication of the abstract
   parameters of the quality of service desired.  These parameters are
   to be used to guide the selection of the actual service parameters
   when transmitting a datagram through a particular network.  Several
   networks offer service precedence, which somehow treats high
   precedence traffic as more important than other traffic (generally
   by accepting only traffic above a certain precedence at time of high
   load).  The major choice is a three way tradeoff between low-delay,
   high-reliability, and high-throughput.

     Bits 0-2:  Precedence.
     Bit    3:  0 = Normal Delay,      1 = Low Delay.
     Bits   4:  0 = Normal Throughput, 1 = High Throughput.
     Bits   5:  0 = Normal Relibility, 1 = High Relibility.
     Bit  6-7:  Reserved for Future Use.

        0     1     2     3     4     5     6     7
     +-----+-----+-----+-----+-----+-----+-----+-----+
     |                 |     |     |     |     |     |
     |   PRECEDENCE    |  D  |  T  |  R  |  0  |  0  |
     |                 |     |     |     |     |     |
     +-----+-----+-----+-----+-----+-----+-----+-----+

       Precedence

         111 - Network Control
         110 - Internetwork Control
         101 - CRITIC/ECP
         100 - Flash Override
         011 - Flash
         010 - Immediate
         001 - Priority
         000 - Routine

   The use of the Delay, Throughput, and Reliability indications may
   increase the cost (in some sense) of the service.  In many networks
   better performance for one of these parameters is coupled with worse
   performance on another.  Except for very unusual cases at most two
   of these three indications should be set.