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.

Total Length

Total Length is the length of the datagram, measured in octets,

   including internet header and data.  This field allows the length of
   a datagram to be up to 65,535 octets.

All hosts must be prepared

   to accept datagrams of up to 576 octets

Identification

16 bits

An identifying value assigned by the sender to aid in assembling the fragments of a datagram.

• Flags: 3 bits
  • Bit 0: reserved, must be zero
  • Bit 1: (DF) 0 = May Fragment, 1 = Don't Fragment.
  • Bit 2: (MF) 0 = Last Fragment, 1 = More Fragments.

         0   1   2
       +---+---+---+
       |   | D | M |
       | 0 | F | F |
       +---+---+---+

• Fragment Offset: 13 bits

This field indicates where in the datagram this fragment belongs. The fragment offset is measured in units of 8 octets (64 bits). The first fragment has offset zero.