RFC 791

图 4：IPv4 首部

    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Version|  IHL  |Type of Service|          Total Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Identification        |Flags|      Fragment Offset    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Time to Live |    Protocol   |         Header Checksum       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Source Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Destination Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Options                    |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Example Internet Datagram Header

                               Figure 4.

  Note that each tick mark represents one bit position.

长度、标志、TTL、协议、校验和

  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.

[……节选 Identification，下接 Flags]

  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 |
        +---+---+---+

[……节选页眉，下接 Fragment Offset / TTL / Protocol / Header Checksum]

    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.

谁切、谁拼、哪些字段会变

    The internet identification field (ID) is used together with the
    source and destination address, and the protocol fields, to identify
    datagram fragments for reassembly.

[……节选页眉。未分片条件见下条注解；此处从完整句开始]

If an internet datagram is fragmented, its data portion must be
    broken on 8 octet boundaries.

[……节选片数上限说明]

    Every internet module must be able to forward a datagram of 68
    octets without further fragmentation.  This is because an internet
    header may be up to 60 octets, and the minimum fragment is 8 octets.

    Every internet destination must be able to receive a datagram of 576
    octets either in one piece or in fragments to be reassembled.

    The fields which may be affected by fragmentation include:

      (1) options field
      (2) more fragments flag
      (3) fragment offset
      (4) internet header length field
      (5) total length field
      (6) header checksum

    If the Don't Fragment flag (DF) bit is set, then internet
    fragmentation of this datagram is NOT permitted, although it may be
    discarded.  This can be used to prohibit fragmentation in cases
    where the receiving host does not have sufficient resources to
    reassemble internet fragments.