Internet Header Format

This description is taken RFC 791, pages 11 to 14.

Example Internet Datagram Header

Note that each tick mark represents one bit position.

Version: 4 bits

The Version field indicates the format of the internet header. This
document describes version 4.

IHL: 4 bits

    Internet Header Length is the length of the internet header in 32
    bit words, and thus points to the beginning of the data. Note that
    the minimum value for a correct header is 5.

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.

    The type of service is used to specify the treatment of the datagram
    during its transmission through the internet system. Example
    mappings of the internet type of service to the actual service
    provided on networks such as AUTODIN II, ARPANET, SATNET, and PRNET
    is given in "Service Mappings" [8].

    The Network Control precedence designation is intended to be used
    within a network only. The actual use and control of that
    designation is up to each network. The Internetwork Control
    designation is intended for use by gateway control originators only.
    If the actual use of these precedence designations is of concern to
    a particular network, it is the responsibility of that network to
    control the access to, and use of, those precedence designations.

Total Length: 16 bits

    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. Such long datagrams are
    impractical for most hosts and networks. All hosts must be prepared
    to accept datagrams of up to 576 octets (whether they arrive whole
    or in fragments). It is recommended that hosts only send datagrams
    larger than 576 octets if they have assurance that the destination
    is prepared to accept the larger datagrams.

    The number 576 is selected to allow a reasonable sized data block to
    be transmitted in addition to the required header information. For
    example, this size allows a data block of 512 octets plus 64 header
    octets to fit in a datagram. The maximal internet header is 60
    octets, and a typical internet header is 20 octets, allowing a
    margin for headers of higher level protocols.

Identification: 16 bits

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

Flags: 3 bits

Various Control Flags.

      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.

Time to Live: 8 bits

    This field indicates the maximum time the datagram is allowed to
    remain in the internet system. If this field contains the value
    zero, then the datagram must be destroyed. This field is modified
    in internet header processing. The time is measured in units of
    seconds, but since every module that processes a datagram must
    decrease the TTL by at least one even if it process the datagram in
    less than a second, the TTL must be thought of only as an upper
    bound on the time a datagram may exist. The intention is to cause
    undeliverable datagrams to be discarded, and to bound the maximum
    datagram lifetime.

Protocol: 8 bits

    This field indicates the next level protocol used in the data
    portion of the internet datagram. The values for various protocols
    are specified in "Assigned Numbers" [9].

Header Checksum: 16 bits

    A checksum on the header only. Since some header fields change
    (e.g., time to live), this is recomputed and verified at each point
    that the internet header is processed.

The checksum algorithm is:

      The checksum field is the 16 bit one's complement of the one's
      complement sum of all 16 bit words in the header. For purposes of
      computing the checksum, the value of the checksum field is zero.

    This is a simple to compute checksum and experimental evidence
    indicates it is adequate, but it is provisional and may be replaced
    by a CRC procedure, depending on further experience.

Source Address: 32 bits

The source address. See section 3.2.

Destination Address: 32 bits

The destination address. See section 3.2.

Options: variable

    The options may appear or not in datagrams. They must be
    implemented by all IP modules (host and gateways). In our
    examples presented so far, no options were used. For full
    description of the options, please refer toRFC 791.