September 1981
Internet Protocol
Specification



    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.






 

September 1981                                                          
                                                       Internet Protocol
                                                           Specification



  Options:  variable

    The options may appear or not in datagrams.  They must be
    implemented by all IP modules (host and gateways).  What is optional
    is their transmission in any particular datagram, not their
    implementation.

    In some environments the security option may be required in all
    datagrams.

    The option field is variable in length.  There may be zero or more
    options.  There are two cases for the format of an option:

      Case 1:  A single octet of option-type.

      Case 2:  An option-type octet, an option-length octet, and the
               actual option-data octets.

    The option-length octet counts the option-type octet and the
    option-length octet as well as the option-data octets.

    The option-type octet is viewed as having 3 fields:

      1 bit   copied flag,
      2 bits  option class,
      5 bits  option number.

    The copied flag indicates that this option is copied into all
    fragments on fragmentation.

      0 = not copied
      1 = copied

    The option classes are: