F.1 IP Datagram Header

This description is taken from pages 11 to 15 of RFC 791, Internet Protocol , by Jon Postel, Information Sciences Institute, University of Southern California.

Internet Header Format                                          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    |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    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.  The minimum value for a correct header is 5.    Type of Service:  8 bits      The Type of Service indication the quality of service desired.     The meaning of the bits is explained below.        Bits 0-2:  Precedence.       Bit    3:  0 = Normal Delay,      1 = Low Delay.       Bits   4:  0 = Normal Throughput, 1 = High Throughput.       Bits   5:  0 = Normal Reliability 1 = High Reliability.       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    Total Length:  16 bits      Total Length is the length of the datagram, measured in octets      (bytes), including internet header and data.    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.  The Flag bits are explained below:        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.     Protocol:  8 bits      This field indicates the Transport Layer protocol that the data     portion of this datagram is passed to.  The values for various     protocols are specified in the "Assigned Numbers" RFC.    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.    Source Address:  32 bits      The source IP address.  See 
Chapter 2, 
Delivering the Data

, for a     description of IP addresses.    Destination Address:  32 bits      The destination IP address.  See 
Chapter 2
 for a description of IP     addresses.    Options:  variable      The options may or may not appear in datagrams, but they must be     implemented by all IP modules (host and gateways).  No options     were used in any of the datagrams examined in 
Chapter 11
.